Allergic to Routine

This person gets bored extremely easily and needs to be entertained at all times. Unpredictability isn't just preferred, it's a necessity for this guy's sanity -- if too many days pass quietly, they start to lose their mind. They're the roommate who always "forgot" that it was his turn to clean up, but who dragged you along on his (mis)adventures and kept life so interesting that you ended up not minding. They're also the parent who uprooted their kids every time they felt life was getting "boring" and the spouse who signed themselves and their unknowing partner up for a mission to Jupiter -- anything, anything to avoid life becoming "normal."

Your sympathy for this character may vary. One one hand, they're funny, unpredictable and keep life interesting...but on second thought, you'd probably think twice about marrying this character or having them as a parent. They often have a crippling case of No Sympathy, and assume what they want is what's best for everyone. The fact that their boyfriend is keen to take over the family business in his hometown, or their kid is sick and tired of having to make new friends every four months, will simply not compute with them -- life's getting same-y, so let's get out of here!

However, they may not get as far as having a steady partner, far less kids, to worry about -- that means commitment, and very possibly the dreaded prospect of domesticity. Many Allergic To Routine characters would rather remain free agents, never hanging around for breakfast after a passionate night or forming lasting friendships. If they do, it will be a major plot point.

They may use dubious means to make things "interesting," or disrupt the lives of those around them just to see what happens. They will enjoy being a Weirdness Magnet, would never dream of declaring "I Just Want to Be Normal" and, if faced with normality, will be spotted running into the horizon screaming "I Just Want to Be Special"...except it's not the "specialness" or Chosen One status they want, it's the ability to make life interesting.

Contrast with Creature of Habit.

Anime and Manga

 * Eroica/Dorian is a less manic version, but still gets pretty low when life fails to live up to his romantic ideals, and sets off to cause trouble in order to keep life exciting.
 * Both Light Yagami and L of Death Note become depressed when faced with monotony. The same goes for Ryuk.

Literature

 * Haruhi Suzumiya, of course. Ordinary day? Heaven help you, Kyon, Haruhi will soon sort that out.
 * Sherlock Holmes: "My mind rebels at stagnation." Gets clinically depressed when things get too quiet. For the record, the book version of Watson has a foot in both camps (domestic bliss with wife+running after Holmes on adventures = happy life), while the film version wants to be a Creature of Habit...but Holmes at least plays on the assumption that his best bud is naturally Allergic To Routine.
 * The modern TV version of Sherlock Holmes has an almost addictive need for mental stimulation and will go to any number of potentially dangerous lengths to avoid getting bored, a trait he shares with Moriarty.
 * The obligatory Discworld example would be Moist Von Lipwig of Going Postal and Making Money. After he outwits the Corrupt Corporate Executive and gets the post office running again, he resorts to climbing walls and breaking into his own building to kill the monotony. Vetinari promptly shifts him to the bank, and already has plans for his next job after that. One wonders what will happen when the city runs out of businesses for him to renovate, although Vetinari also notes that he doesn't seem to get bored like this when his fiancée is around.

Live Action TV

 * In Doctor Who, the Doctor is terrified of domesticity and monotony - after all, this is the guy who regularly switches bodies, never mind routines. This has caused some heartaches for his companions, who occasionally and understandably feel as if they're just replaceable amusements who get discarded when the Doctor gets bored.
 * Given that he's gone through 11 bodies in the past 900-odd years, he actually keeps them slightly longer than the average human.
 * Whether as a doctor, a friend, a roommate, Gregory House always winds up rewriting the rules to his own advantage--and sometimes breaking them even then. Not too surprising, as he's a Sherlock Holmes Expy.
 * The Fall and Rise of Reginald Perrin: Reggie is being driven mad by his routine. Especially the season-spanning routine of destroying everything he's ever created.

Web Comics

 * Eight Bit Theater: Black Mage goes stir crazy very, very quickly.

Real Life

 * In astrology, Sagittarius and Aries. Aquarius is an odd case - most descriptions will paint them as apparently Allergic to Routine, but actually a Creature of Habit - it's just that those habits will be very strange.
 * In Myers-Briggs, Intuitives and Perceivers are usually more routine-phobic.
 * This is a disproportionately common trait in people with ADHD.