Infectious Enthusiasm

This trope is most common in musicals, but is not strictly relegated to only musical fare.

You have a happy go lucky or other highly charismatic character who is getting everyone around him/her all excited about whatever it is they're excited about.

Then there's the one character who isn't swept up. They're worried that everyone is all excited and not thinking it through. They are concerned that there's something shady under all the happy excitement. They're just grumpy and think all this excitement is too silly and twitterpated.

But as the song/scene goes on, the excitement spreads, and the enthusiasm grows. The lone holdout will find themselves going along and either catch themselves with embarrassment and stop—or will not catch themselves and end up going with the flow.

Compare with Not So Above It All.

Examples of Infectious Enthusiasm include:

Anime and Manga

Aversion: In Mahou Sensei Negima Chisame when everyone is making a Hooray for [Grinding] until she starts thinking of them as friends (With the [[[Heroic BSOD]]] Chapter 176 pages 04-05.

Film

Enchanted: Giselle starts singing "That's How You Know"—Richard is embarrassed and annoyed. Then he's shocked when the local musician buskers join her. By the time Giselle has gotten most of Central Park involved, he's sarcastically playing along. By the final chorus, though, he's smiling and bobbing his head along with the tune, until he catches himself with a mortified "what the hell am I doing?!" expression.

Literature

Kamina appears to have this power (as noted by the author) in Double K.
The Shove in Unseen Academicals: a single moment where all humans are united in a single emotion (to the point of creating false memories), including the wizards unused to such emotional outbursts.

Live Action TV

On How I Met Your Mother Barney often gets really excited about some new thing, and will spend a lot of energy trying to get his friends to be just as psyched about it. This only works about half the time, though.

Web Comics

This is a staple of people with The Spark in Girl Genius.

Western Animation

Played with in Avatar: The Last Airbender: "The Fortuneteller". Fun-loving Aang enjoys Aunt Wu's fortunetelling and hopes she'll predict he'll end up with his crush. The usually level-headed Katara is also swept up in the Aunt Wu fortunetelling mania. Which drives rationality-and-science-loving Sokka up a wall—he keeps trying to disprove it all as bunk.
- And in The Legend of Korra: Tenzin disdains pro-bending as a mockery of the noble history of bending. Until he sees that it works to teach airbending basics to Korra where he has been unsuccessful, and further, that it helps her team win the match. At which point he is swept up in the excitement of the jubilant crowd and cheers himself! Before collecting himself...and then going on to become an actual fan of the sport!
My Little Pony Friendship Is Magic: "The Super Speedy Cider Squeezy 6000": The Flim-Flam brothers' song, "Opportunity" causes everypony in Ponyville to get excited and start chanting "Cider! Cider! Cider!". Early in the song, Twilight and Spike stand out as watching this whole thing play out, wearing dubious expressions. By the second verse, they are still looking concerned. But the next time the camera returns to them, they're wearing big smiles and chanting "Cider!" along with everypony else.
The Simpsons
- It is subverted in "Monorail". Marge remains worried and uncertain and gets mocked for it.
- "Lisa the Vegetarian": Homer & Bart are mocking Lisa's newly found vegetarianism on the eve of Homer's BBBQ. Bart starts a chant "You don't win friends with salad!" Homer joins in creating a conga line. Then Marge joins.

Lisa: Mom!
Marge: I don't mean to take sides, I just got caught up in the rhythm.

A fairly common occurrence in Phineas and Ferb:
- In "Tip of the Day", the eponymous characters sing a song to raise awareness for aglets. Candace is not impressed as usual at first, but as the song goes on she gets increasingly enthusiastic.
- In "Road Trip", Candace tries to bust the boys for starting a truck stop on top of the family's rented RV. She takes up the waitressing grudgingly. By the time they've gone through the song, she is looking at the order turnabout and herself like "what am I DOING?!"