1.) I'm partly motivated by fear: Extensive cardiac disease in my family, coupled with every male in my family getting diabetes by age 50.
2.) Partly motivated by mood: Depression runs in the family, and I notice a distinct change in my "happiness quota" if I go for a few days without popping a sweat.
3.) But mostly motivated by the sheer joy of the process. I've grown to love challenging myself, the sense of satisfaction derived from a great workout.

4.) It's become "What I do." Recently I was griping that I was running low on free time. I do spend ~1 hour at the gym 4 times a week. But what would I trade that for? More time sitting on the couch watching TV? That's no fun. And TV time doesn't reward me anywhere near as much as gym time. I'd rather be an exercise guy than a TV guy.
5.) I like looking better nekked.
Have fun,
Cliff