Being a pessimistic hero can be an enormous problem at times.
You want to beat the villain, but you're so certain you won't, so why even try? You'd train your superpowers, but in your mind you're already the weakest superhero in the bunch, so why bother?
But when the villain currently threatening the world has the power to turn one's optimism against themselves... Well, then you're suddenly an opponent to be reckoned with.
And when said villain's weakness is snark and sarcastic comments, it's finally your pessimistic self's time to shine.
Don't forget to pack your self-doubt and crippling worries when you leave, though, or you might still lose.