And do you exclusively GM, or do you play as well?
It's a simple question, but since I pondered it for a post my brain has gone for quite a while without coming up with a satisfactory answer for me. It's not that there are a lack of GMs in the area - in our group of 7-9 regular players, 5 of us are active GMs.) So, why do it if it's not for the need?
Part of it is I like GMing to tell stories. I mean, sure, you get to tell stories as a player too but as a GM you get to tell big stories. Better yet, you get to craft and mold those stories to give your players awesome moments where they get to shine, be heroes, do despicable things, redeem themselves, and almost everything else that can be conceived. It's an awesome feeling when the magic really hits, and you get to feel that magic more from the GM's side of the screen because there you are a part of every story at the table, not just the ones that include a specific character.
Beyond that though, I think I like GMing in and of itself. As a mental exercise GMing has a lot going for it. You craft worlds, stories, characters, enemies, monsters, puzzles, obstacles, the whole nine yards. You are force to improv not just one character, but a whole world. You have to have plans strong enough to make sense and work, but flexible enough that a PC can just decide to go left (or fly off) at any point in time.
I like, maybe even thrive, on the fact that the players in my game not only trust me to run good games but actively set parts of their lives aside to play. Just try telling yourself you suck when someone is driving 90+ minutes each way just to sit at your table. Sure, they could be coming for other reasons, but you don't make that trip if the game isn't worth it.
So why do you GM? Not what got you started, but what hooked you and stapled your butt to that chair?