It can be something as simple as inherit from a relative, or maybe a personal connection with a super hero. For me it's a mixutre of love for the stories. And the idea that the heroes fight for something greater then themselves. I also like the idea of a superhero that is impure, (I feel Marvel does this better, why I hate heroes like Superman and Batman). I like how comic books show that they have a human side to them, and are not perfect.
What about you guys?