Batgirl

"Barbara: I'm strong enough and smart enough for anything you throw at me.
Batman: Then answer one question. Why?
Barbara: Well, I... I...
Batman: I thought so. This isn't a game. Time to go
home, little girl.
Barbara: Is that it? I fail the final because I can't tell
you in fifty words or less why I want to do this? You
may have all the tools of the trade, but you don't have
a monopoly on wanting to help. I'll tell you why, you
big scary goon... because I can."