I enjoy GMing, and so I want to do it.
Most groups I have GMed have wanted me to DM and specifically asked me to do it.
Less nobly, most groups I have been involved with, the GMing ultimately frustrates me as being inferior to what I know I can offer and what I would be happy with as a player. I find that I enjoy the game more being the GM even when I would prefer to be a player, than I would being a player with a GM that lacks the skill or experience to do the job well. This bit of arrogance on my part typically gets bolstered by the fact that players that I have had typically are disappointed by the games run by other GMs and tend to beg me to run a game.
I do get to points where I really miss being a player and being able to go to a game without having spent my whole week planning for the game, and to just show up and do role-play with likeminded players who also can role-play. Playing is also a lot of fun and I haven't got to do much of it in like 25 years now.