Like, something personal and life altering that connects them to their decision to take up the role that they did. More than "Well I have these powers," or "I got rejected."

Consider if Professor Oak was acting as a villain as a means of protecting the earth and environment. Yes, in the eyes of society, he is "evil" but the fact is, he's a hero in his own eyes.

That's always the mark of a good villain! A villain shouldn't be a villain for the sake of being evil, but for a cause that is dear to their heart. They're fighting for something, something important to them that they aren't willing to abandon even at the cost of the lives of others or even themself.

The same can be said for a hero! They shouldn't be a hero just for the sake of being a hero. They should have something that drives them. Maybe the fire chief died, or was injured in one of the fires and the police doing nothing about it was a personal sting. Even heroes can be driven by revenge! Maybe the bugs want Cricket to protect people because humans play a big role in bug survival from planting flowers that bees pollinate, to leaving food for ants and other species, to their corpses being consumed by worms and the like! There's plenty of ways to go with it!