A brief list...
1) I have to for my job. We have run requirements at work I have to meet.
2) I really do have to for my job. If I don't run, I don't exercise and I have to stay at or below a certain weight for my job, and I could stand to lose a bit.
3) I have to for my friends. There was a time when I didn't run and work was super stressful. My friends told me later that they didn't want to be around me because I was so moody. Since I have started running my outlook on life has improved and overall I'm a happier person.
4) I have to for my sanity. I used to go home from a stressful day at work and sit in my chair on the couch and just stew over everything that went on during the day, or I'd vent to my friends which I would feel guilty about later because they don't care about my little piddly problems. But when I run I can work out and stew while I run, which gives me momentum and a way to work out any issue I might have before I blast it at a friend or loved one. It also helps me stay calmer than I would be otherwise. Of course, now that I run I don't have as much stress from work to even stew about! :)
5) I need a goal. Because of #1 and #2, I have to have a goal to keep running. I'm really good at just sitting on the couch and not doing anything, but with a goal I have a little nudge that keeps me going. So I sign up for races. I know I'm not going to win, but I am competitive enough with myself that I will push myself to compete and complete it (and not be last, except that one last year). But I'm also not one to just sign up for a 5K if a 10K is offered (not anymore at least). I can't sign up just for the 5K St. Patty's Run when there's an Irishman to complete. I can't sign up for a 1/2 marathon if there's a full offered. It's that competitiveness in me. Or stupidity.
6) I do love to eat. With the amount of baking and taste-testing I do, if I didn't run I'd be 300 pounds. I'm not a stick by any means, but I do hold my own. I could stand to lose a few pounds, but who couldn't? But by running consistently, when I do stray and have a cookie or two (or six) I don't feel quite as guilty as I used to. And my pants continue to fit.
7) I feel better about myself when I run. Granted after I run I'm all bloated and red-faced and feel tight and sore for a few hours, but the accomplishment that I have completed - the miles I have logged (and you must log them...well, I must) - that's the great feeling that keeps me going.