I enjoy GMing, but I prefer playing as a PC. So when our regular GM's taking a break, wants to simply be a PC to avoid burnout, or the group as a whole is itching for multiple sessions a week, I put on my GM cap.
I really enjoy the act of GMing, and I like telling interesting stories, it's just that I don't like all the preparation and pregame work that's required for it. If I was a better improviser (and I consider myself a fairly good one) I'd be more comfortable with it, but I always feel like I'm letting my players down when I don't have a full adventure prepared, and so even when I'm doing well, I tend to doubt myself. I suppose I could get over it with more practice, but I simply like the no-worries feeling that comes from being a player. After all, I'm only personally responsible for MY fun that way, and I don't need to worry myself about everyone else's good time (though I do try to be conciencious)