1 - the creativity of it
2 - being able to entertain others
3 - nobody else in the group will do it
While the first two will virtually always be more than enough motivation to run a game, the third is actually a consistent drain. I love to be a player as much as the next guy. MORE really. I too love the creativity in developing a single character, interacting with other PC's, collecting loot, planning my castle, my own guild of thieves, my temple, etc. But it is a different experience than running the game because it directly reverses what I do and don't control about the game.
If I had my way I'd be playing D&D the way I played it back when I first started playing - every Saturday, almost like clockwork, for 8 hours or better. I first started to DM for the additional creative outlet, but there came a time when all the others in the group who DM'd were more willing to NOT game at all than to expend the effort to run their own games. So, if ANYONE in the group was going to play, it would only be if I ran the game. I wanted to game more than they wanted to be DM's any more. Eventually, that made it a chore because I never got to experience the other side of the screen anymore. I wanted that same players experience now and again but nobody would provide it anymore. That in turn meant that I never got a break to RENEW my creativity as a DM - and that resulted in burnout and no gaming for anyone of any kind.
Now I'm not saying anyone has an OBLIGATION to DM any more than a player has an obligation to play, just that the lack of others around willing to run their own games rather sucks for me, and it has a knock-on effect upon my players.