[I posted this a long, long time ago, but then I neglected to create the page. Not sure if something similar has been done since.]
You got all dressed up to fight for the causes of truth, justice, and a happy ending for all, but nobody invited you to the party, because you're incompetent, socially maladroit, unsightly, always in the way, or otherwise damaged merchandise. It's also possible that your over-the-top enthusiasm makes the heroes vaguely uneasy. Either you creep them out by trying to hard to fit in, or they suspect you might just be taking the piss. As a result, they blow you off, causing you to rechannel your irrepressible ardor and not-inconsiderable powers into demonstrating to them just how big a mistake this was.
Your motivation can add a sympathetic dimension to an otherwise irredeemable character, who started with the best intentions, and only wanted a bit of recognition. Meanwhile, your ill treatment can rub some of the gilt off an otherwise all-too-saintly protagonist, who could have headed off your rampage if he had only given you a pat on the head, and offered you a bit part in his parade of do-goodery and pithy one-liners.
A few examples (spoilers for The Incredibles, 300):
Syndrome of The Incredibles, the hunchback of 300, Darth Vader, the Greek god Eris, whose golden apple led to the Trojan War, and the snubbed fairy who, in retaliation, gifted Sleeping Beauty with a curse, rather than a blessing.
Real-life examples might include any number of social outcasts who show up at school one day and gun down the popular kids, as well as scorned suitors who take revenge upon the prior object of their affection.
The opposite of this trope would be Rudolph the red-nosed reindeer, who demonstrates that his ostracism was a mistake by actually doing good, rather than making life more difficult.