I ask not because people haven't thought on this, but because we don't return to it enough. Between all the models and rubrics and student performances and stuff that we complain about because it really does suck, I think we can lose sight of it.
I teach because since as long as I can remember, I've been unable to excel at challenges for my own benefit--I've always been more the mom who picks up the car to save her child than the athlete who outruns everyone to glory. I teach because it's the only non-ephemeral human exploit, so I figure it must have some divine significance, or at least some lasting meaning.
Most of all, I teach because if I don't there's still going to be 120 kids sitting in my classroom without a teacher to support them, and I know that I'm not going to "save" them and it won't always go well, but their lives and mine will be better for our interactions.
Why do you teach?