Happy Tree Friends/WMG

The constant death/revival cycle is due to Sniffles constantly rewinding time.
Exhibit A: Blast from the Past.

The HTF Universe is really a hallucination
Think about it: each character represents a different person. They are all (except for Cub - he just happens to be with Pop.) taking the same drug. The universe and their forms represent their imaginary world-their deaths represent them suffering from abuse of the drug in reality.

The show is actually a Show Within a Show itself.
Excuse the Mind Screw in the last sentence.

In a few cases, the HTF videos are educational (e.g. Copyright School), but most of the time, it's rather comedy with a slapped-on Aesop at the end that may or may not make sense in context. That reminds me of Executive Meddling... The theory is that, rather than the episodes we see being their "reality", what we see is something that the character-actors within the show have recorded. Therefore we now have a few explanations - all the deaths are done by loads and loads of editing, they "return" because their actors return to their roles...

Happy Tree Friends takes place in Purgatory.
A huge disaster caused by a single mistake killed all the forest inhabitants, cursing their souls to remain in Purgatory until they can break the cycle of violence and destruction. Only when they can get through a day without horribly slaughtering each other can they pass into heaven.

Happy Tree Friends takes place in Hell.
The frequent wholesale slaughter is the result of the twisted games of some bored but omniscient demon.
 * Or how about a bored witch, right, tropeeeeeeeeers?

The town where everyone lives is a Genius Loci very similar to Silent Hill
Except instead of "playing with" troubled visitors, it does it to its own inhabitants. It's also able to warp reality like the other one, but while Silent Hill uses it's power to create a nightmare version of itself, THIS town uses its power to continuously revive and wipe the minds of the citizens so they can be killed again and again; as well as cause everything that CAN go wrong TO go wrong, which is what ends up killing everyone every time. Much to the amusement of the town.

The entire series is in Flippy's head.
Flippy's in a mental hospital somewhere, imagining the entire thing. Would explain why everyone dies so much, only to come back, good as new the next day: Flippy was in war, and apparently, war and how bloody it was is all he can think about now. It also would explain why Flippy could be out and about with his mental state.

Happy Tree Friends takes place in an overlap world between Higurashi and Death Note.
Kira hates furries, so he slaughters them in increasingly more gruesome ways in frustration that they keep coming back when the Groundhog Day Loop resets.
 * Jossed. The Death Note rules specifically state that the cause of death listed must be physically possible, something that Happy Tree Friends isn't particularly concerned with.
 * Although "physically possible" could just be more loosely defined here...

Cub's mother/Pop's wife made a Deal with the Devil.
She sold her soul to make sure her son and husband (as well as the rest of the cast) always come Back From the Dead.

The HTF gang are either in an RPG or a tabletop game and can be revived.
The characters come back to life each time round, and Lumpy takes some care trying to retrieve all of the corpses in Take a Hike, so maybe the characters have been "just knocked unconscious" by all those horrible incidents and Lumpy was trying to get them to the local doctor for rezzing before a Total Party Kill occurred. Furthermore it would explain the insane risks Made of Plasticine characters take, and the fact that they can survive apparently fatal injuries; for them, its just lots of lost HP. No prizes for guessing what the GM is like.
 * Leading on from that, what would the characters stats be like? Low wisdom stats all around naturally, though not necessarily low intelligence. For example, Sniffles is smart enough to assemble advanced technologies, but the things he does with them... Lumpy though... I guess he would have the highest charisma seeing as he always seems to end up in a leadership role despite his being The Ditz, but he's clearly the dimmest of the bunch.
 * Perhaps they're in a hyper-violent stress-relief game of Toon?

The characters in HTF are Time Lords/Ladies.
The reason they can die horribly in one episode and be back to die horribly in the next episode is due to regeneration. None of them are in possession of a working Tardis.
 * But if that were true, they wouldn't look the same in every episode.

The Happy Tree Friends can all regenerate.
Dying horribly is pretty much par for the course for them.

Happy Tree Friends takes place in the same universe as Higurashi no Naku Koro ni.
... except the interval of the loops is shorter.
 * Why would it loop? Witches. Though which one of the characters is a witch/sorcerer isn't clear.

The series is a more "ordinary" Groundhog Day Loop than the ones mentioned above.
Higurashi and Umineko are murder mystery-series (no matter what fanopinion has to say on that matter) and you find yourself asking "How could that kind of death happen?" when watching HTF more often than "How/Why did (s)he die/get killed?"...

The episode Without a Hitch takes place in a rare world where Flippy never suffered from PTSD.
There are several things that occurred in that episode that should have have made him "flip out" under normal circumstances (the car crash, getting stabbed in the freaking eye, etc.), yet he didn't. Furthermore, Flaky's recurring visions of Flippy murdering her in horrific ways could be explained by her remembering bits and pieces of past worlds a la Minagoroshi-hen, by her having an advanced case of Hinamizawa syndrome (note that symptoms include fucked-up hallucinations of people with creepy eyes and paranoid distrust of people one normally feels safe around), or a combination of the two. She would have likely died via heart attack or slitting her throat with her nails had her airbag not deployed and splattered her. Alternatively, perhaps she did die this way; the airbag was yet another hallucination. Of course, the chances of Flippy being free of his mental affliction were one to a million; once Rika (or Happy Tree Forest's equivalent of Rika, I'm really not sure) moved on to another world, the old Flippy was back.
 * Another possibility: Flippy doesn't flip because his PTSD has been, if not really cured, at least weakened. Flaky is believed to be one of his best friends, after all. He can still flip, but it's harder and the flipping is much less intense. More on this further down in the "character"-section.

The death of Cub's mom/Pop's wife is what starts the string of deaths in every time loop.
It makes sense considering that, if the hints are to be believed and she is truly dead, she's the only character who stays that way. The reason the Happy Tree Friends are doomed to suffer and die again and again is because Pop always fails to save her. Pop's wife died in the same accident that Handy lost his hands in, which would explain why he always appears as an amputee even when worse injuries to other characters seem to have no lasting effect.

The Idol causes all of the deaths (unless they are murders, when it just makes them much easier).
It shows up too often as a background cameo for it to have no effect whatsoever. Perhaps there are more than one, too...

Pop's wife/ Cub's mom almost always finds the idol and gets killed by it.
The exception is Idol Curiosity, which is the only time so far where it has showed up in a place that made sense.

Cub controls the time loops.
He travels through the time-space continuum indefinitely, trying to find a world where his father is not a bumbling moron and his mother is still alive, and he must start over again every time Pop gets him killed/somebody else dies.

Alternatively, Flaky controls them, or atleast can remember the past loops.
Which is why she is so cautious: so she won't screw it up.

... and the loops are already over.
If you combine the above theories with the lower of the episodes being in non-chronological order, this could be possible. And now We Wish You and Asbestos We Can Do are less time-wasting and more like short snippets of Slice of Life to show us that they did get their happy ending... even though we might as well watch more past loops for kicks.

Flippy is partly cured of his PTSD as of Double Whammy Pt. 2
Depending on whether or not psychological changes span episodes/death in the HTF-verse. He was shown to be cured of his Split Personality at the end of Double Whammy (before promptly being run over by a semi), and this has since been carried on into Without a Hitch. In Without a Hitch, even though Flaky imagined him flipping out three times, he never actually flipped out despite being in the presence of pretty good triggers: loud thunder, a car crash, and a bloodied Flaky. Giggles covered in ketchup was enough to set him off in Flippin' Burgers, so a badly injured Flaky should have made him flip out, if anything.

Flippy's PTSD hasn't been cured yet.
And not just because the latest Love Bites seemingly Jossed it either. The primary proof for this is refuting the primary evidence used to say he has: Without a Hitch. But don't worry, Flippy x Flaky shippers, 'cause I don't think this sinks anything.

With the exception of Flippin' Burgers, all of Flippy's flip-outs have a similar cause: events that remind of him of gunfire or fire in general. In short, they remind him of the enemy and being under attack. As a Shell Shocked Senior, this would cause him to subconsciously switch to "kill-mode" in order to survive the oncoming battle.

In Without a Hitch, no such trigger poses itself. Although on first glance, one would think the sight of the wounded Flaky would set him off, it doesn't... because this isn't a situation that would require him to flip out. This brings up a different set of war memories: the wounded ally in need of medical attention; never leave a man behind. The original kind, caring Flippy is the one needed here and not the id-riddled killing machine, so he's never triggered in the usual way during the course of the episode.

The only exception, as previously stated, is Flippin' Burgers, where he flips due to seeing Giggles "blood"-stained (though in that case she was "dead" while Flaky was just injured), though there are two possible explanations. One, in a similar situation during the way which Flippy is remembering, there were enemies near the scattered corpses, which would be a trigger for "kill-mode." Or two, it indicates that while Flippy isn't cured, he's getting better.

Russell's past isn't as happy as his personality makes it seem.

 * 1. He's somehow lost his hand, eye and both legs. That goes beyond mere unfortunate clumsiness, even by HTF-standards.
 * 2. He's gone crazy at least once (after killing Lumpy by mistake in Get Whale Soon). Judging by his way of reacting to that, it was as if he got a flashback of something terrible and snapped. (One could argue that it was because he likes Lumpy more than the fans do, or at least would prefer having him as company rather than being alone whilst the juices digested him...)
 * 3. Where on earth are the other pirates, and why does he live on land (evident by Sea What I Found)?

It could be that Russell used to be a pirate on a large ship. One day, they were attacked by another, more serious (being type 1 rather than 2) group of pirates, who killed almost everybody from Russell's boat who didn't want to hand their loot over. Russell and a few others who weren't killed already tried to flee, but before they could manage, they were caught, tortured brutally and thrown off the boat. Deeply scarred, Russell had to use his friends' equally mangled bodies to reach the shore, but fainted before doing so. He was washed up on a shore close to where the HTFs live and recieved treatment. Thankfully, the nature of his damage before passing out didn't make him remember exactly what happened. Not sure where he had ended up, he tried to start a new, satisfactory life.

... Emphasis on tried.

The idea of Mole being - or rather, have been - a secret agent wasn't completely scrapped.
In Mole in the City, he's seen doing things that might seem a bit odd - actually knowing that he went past the room he should go to, for example, and the audience being allowed to watch through his POV at one point. Well, it could just be a spinoff, although The Mole manages to break himself and Handy out of prison in Don't Yank My Chain. Note also that Splendid has shared jobs with Mole once... Splendid only has this job in order to hide his true identity.

The Mole used to be a slightly clumsy agent, who decided to disguise himself as blind in case any enemy government started to get paranoid. His clumsiness had a price, however: as he put his "mole" back on and it started blinking in the end of Mole in the City, the little "poof!" blew part of his face up and rendered him blind. He couldn't have his real job as an agent anymore, so he became his mask and move to the outskirts of whatever city the HTFs live in.

Along with the above, this explains how he can be seen doing jobs that require sight - he kept them from the period when he hid his true identity.

The Mole is a sadist.
He might not be able to see anything, but he's not deaf. Take a look at Wipe Out, where he makes sure that his surfing board is in good condition. Even when Lumpy gets onto the board. Skin feels rather different to a surfing board, and he should have heard the screams... We have a similar problem in Every Litter Bit Hurts, where Giggles gets impaled and the Mole takes her heart out like it's trash. Adding this to his habit of eating things he shouldn't eat, One can only imagine what his human self would be like...

Nutty had an incident a long time prior the events of the series...
Where he somehow lost his eyes. However, when he was supposed to get replacements, they didn't really have any real ones to donate, so Sniffles (or somebody as smart as him) had to create new eyes for Nutty. Problem was that, due to a mistake, candy got involved by accident in the creation of his eyes. When they were inserted and somehow worked (this series ain't too realistic, deal with it) the brain also got affected, thus building basis for his way too sugary daily life. This would be a way to explain how Russell, after slicing Nutty's eye off to use as a contact lens, saw the kite's string in the same way as Nutty did - the problem wasn'T in his brain, but his eyes.
 * ... however, it seems as if Russell wasn't able to handle this crazy eye, and he died shortly afterwards (shown with the fade to red).

About Cro-Marmot...
The Cro-Marmot that we see in ''Dino-sore Days isn't him, but an old ancestor, given the slightly different appearances. Cro-Marmot used to be a normal Friend, possibly about the same age as Disco Bear, or somewhat younger. We do know that he was successful in many areas, including the piano - he might simply have been interested in history and based his Artist's Name on his favourite historical period. Then somebody got jealous of him, and decided to give him a Fate Worse Than Death (although this may have backfired, given Marmot's smile.

Flaky has a picture on her watch where he isn't encased in ice. *slams desk with hands* This is an indication that he wasn't in ice when/after cameras where invented!

He may also be able to communicate through the ice on his block getting melted into the forms of letters, given how the Interviewer in Intimate Spotlight isn't able to talk with him traditionally, and he yet can get a date or two.

Flippy actually likes Flaky that way.
The fact that Flaky is the only one who is apparently getting along with Flippy (when he doesn't flip out, of course), that he constantly follows her in Without A Hitch just to help her, even though it's obvious she is scared like hell, and that she actually organizes a birthday party for him are some of the factors which imply Flippy and Flaky might like each other. Might.
 * Shippers assemble! I've been working on this theory for a long time. Besides the proofs you put, I'd like to point out another two. Remember Double Whammy? The first time Flippy flips out, he kills everybody and bathes in their blood. The defenseless Flaky comes by and screams, causing Flippy to go back to his usual self. Why would Flaky's screams wake him up? Also, in Party Animal, there is a moment where the flipped out Flippy is looking for prey. He sees poor Flaky inflated because of her allergy and thus unable to move. What does he do? Ignores her and keeps looking for victims.
 * Flippy is known to come out of his flips if he hears loud noises. And maybe he didn't attack Flaky in Party Animal because she looked nothing like flaky. Flipqy consciously killed Flaky 4 times anyways.
 * Yes, but that was in the beginning of Happy Tree Friends. In latter episodes, he apparently gets closer to Flaky (the signs that they're becoming really good friends - Flaky even might be Flippy's best friend- are mentioned by the first two fellow tropers), which determines him to have feelings for her, feelings that might interfere with Flippy's instinct to kill her. There are two episodes in which he doesn't kill her actually: Party Animal and Without A Hitch, even though he could have easily done it.
 * I already explained why he didn't kill Flaky in Party animal. And Flippy might have been cured in Double Whammy part 2.
 * He's not. He flips out in Hear Today, Gone Tomorrow, an episode that aired much later.

The real reason Flaky is afraid of baby chicks...
...is because the only part of  she caught was the end. She likely fainted and missed the rest after.

Flaky is a Tomboy.
There's the complete lack of Tertiary Sexual Characteristics that's characteristic of the female characters but her voice still sound very feminine (well, compared to the slightly deeper male characters and the confirmed female characters), she's thought of as female by most of the cast and fans alike, a Smoochie shows she likes baseball, and we never see her do anything considered girly in the show unlike Giggles, Petunia, and Lammy. All of that sounds like a tomboy candidate. The only real note is that she would be an example of a very rare shy, cowardly tomboy in opposition of the more traditional bold, boisterous tomboy.
 * The creators stated that she is a girl.

Flaky is a MTF transexual
For similar reasons above.

Lammy has mild reality warping powers.
She can turn any pickle she sees into her imaginary friend, Mr. Pickles. As seen at the end of her debut episode, she's in prison and is served her food, which has a pickle in it. The pickle is seen inanimate for a second, then it changes into Mr. Pickles. This power also the one that give Mr. Pickles life and intention to causing harm on the others.
 * I assume this is meant to justify Mr. Pickles's arrival in the prison given the extremely low odds that it's the same pickle she started with?

Lammy will never be used in another cartoon, ever.
She ends her first episode alive and (aside from some Eye Scream) well in a prison for a reason: so she can't get out.
 * Unless she imagines that Mr. Pickles is trying to kill her and thus commits suicide... Then she'll return.
 * Jossed: Lammy appears again (with Mr. Pickels) in "The Chokes On You," but it's only a background cameo. And then she gets another full-fledged appearance in Royal Flush.

Alternately, Lammy is intended to be the new Flippy.
If Flippy has been cured of his PTSD by the end of Double Whammy, then the HTF universe is one insane serial killer short; someone new needs to be added to restore equilibrium. However, this carries the risk of causing Lammy to become a Replacement Scrappy due to Flippy's popularity with the fandom.
 * And if she shows up again, her eye will still be all swollen up (think about Handy's lack of arms).
 * Jossed: Flippy still isn't cured of his PTSD as seen in On My Mind. Lammy shows up again in "The Chokes On You," but her eye isn't swollen there. And if Royal Flush is any indication, Mr. Pickels might be a little more than just a hallucination...

Happy Tree Friends is an example of a typical animated series in the future
Think about it: Recently pretty much all animation has been shifting towards becoming more and more gory, with less clever jokes and more characters having violence inflicted on them with genuinely gory effects, instead of cartoony injuries. Examples would be SpongeBob SquarePants, or The Simpsons. In 20-30 years, all animation will have become identical, removing any attempts at comedy besides horrible injuries inflicted on innocent people. In that time, a show called Happy Tree Friends was created, and it was the most popular show ever, eclipsing everything else. However, one person built a time machine and brought the show into the past in the hopes of becoming rich and famous. The result is the show Happy Tree Friends existing in our timeline.
 * Interestingly enough, this could result in a Stable Time Loop, where the popularity of Happy Tree Friends in this timeline further pushes animation in the gore department, resulting in the "original" version of the show being made in the future.

The episodes are not in chronological order.
Here are a few examples of my hypothesis:
 * In Take a Hike, Flaky tries to help a baby chick back to its nest, but fails and gets killed by the mother. This makes little sense, since From A to Zoo established that she is afraid of chicks. However, if the former episode occured before the latter, this all makes sense: Flaky was killed after trying to help a chick, and is now afraid of them.
 * Or she got over her fear.
 * The episodes where Giggles flirted with other characters, like Blind Date and Giggles' Love Bites, occured before the episodes where she is in love with Cuddles.
 * She could love a lot of characters.
 * This could also mean that Flippy is cured after all.

We don't have a Flippy vs Splendid episode yet...
Because if such an episode where to be released, the amount of Pure Awesomeness generated by it would cause the world to explode. Mondo Media is currently looking into ways to keep the damage contained behind the fourth wall.

Those are all nicknames.
A lot of those names would never show up in real life... And those two with realistic names could easily have moved to the neighbourhood only a short while before the deaths/loops start happening. Lammy may have lived there since her birth, but could be shy about newcomers and rarely get the guts to get to know them.

Lumpy planned to have Mummies At the Dinner Table in the Kitchen Kringle.
If the gas leak really was what offed the other friends, he should have been the first one to go down, as he was in the kitchen himself. We can also only see the gas from a short distance from the kitchen, indicating that it wasn't a lethal threat for Cub and Giggles yet. We also have the issue of the Shlifty twins being there (according to Word of God, although we only see one of them ), despite it being quite clear that Lumpy doesn't like them.

Several of his deaths occur when he's guilty as hell and not just a Lethal Klutz (in those cases, he most often survives), and guess what - here he caused an explosion that killed himself and probably some other poor sods who were walking in the forest, just after he started to laugh maniacally...

He started with slipping a lot of sleeping pills into their drinks. The first ones to go were Giggles and Cub, as Giggles was in a couch (indicating that she felt tired and decided to take a nap), and Cub could be playing a bit before falling asleep. Petunia, however, was determined to go and prepare the table, no matter how tired she was feeling.

Cuddles and Toothy hadn't drunk as much as the others, and only felt sluggish when Petunia suddenly collapsed in the doorway, making them really scared. Lumpy, upon seeing that they would flee and probably report it if he didn't stop them, strangled Toothy and Cuddles, and then made them sit at the dining table after their deaths. Meanwhile, the twins had come to steal some things (no, they aren'T the smartest of thieves). Suddenly when they tried to get away, they found that every door was locked and they couldn't get out before they were caught and met a terrible fate... Lumpy didn't even bother bringing both of them to the table, but dumped one of them there, and then went back to the kitchen to prepare the dinner as if nothing had happened. The gas leak prevented that from happening.
 * As an alternative, he didn't intend to. He could have poured the pills into the liquid by mistake as a result of his stupidity. He could've been just as terrified as anybody else when Petunia fell, and upon checking her pulse, would found that her pulse was so slow that she was beyond help, making his mind start cracking. He could have been determined to keep things as they once had been, with him and the others sitting around the table, no matter the costs. He could have tried and tried to keep Cuddles and Toothy in the house peacefully, but maybe they fell backwards onto something... The twins might have suffocated by the gas if they went in through the kitchen window. The gas could have started to leak by then, and Lifty, who had gone around stealing everything but the cold chicken, suffocated, while Shifty, who had been waiting outside, shut the window after him and left. When Lumpy came back, he threw Lifty out and tried his best to warm the chicken up. Just as he had lit the fire, he came to think of the gas in the room, and was very happy for this nightmare to end when everything blew up.

The Happy Tree Friends are made of candy.

 * But apparently not hard candy. Evidence: Class Act, where Sniffles gets partly eaten by Nutty who has chocolate stains all over his mouth afterwards.
 * ... Oh great, now Nutty's more of a cannibal than before.