I do it for a multitude of reasons.
The biggest being that I've never quite liked myself. Whether my life is plain and boring, or if it's because all of the assholes I've grown up with in this town, I've just never felt right in my own skin. But when I hit the theatre, if I'm onstage doing warm ups or up in the light booth working the lights, I feel reborn. I feel like I've been ripped out of a too-tight outfit, and that I can move freely and with out criticism.
And also, it puts my "mask" to work, so to say. Instead of just pretending to be soul-crushingly happy in the real world, I can exercise that mask, contort it into all the emotions I feel but can't show.