Because I can't imagine not doing it.
I remember when I was in 10th grade, and I started running cross country because my father insisted that I had to do a fall sport (because otherwise he knew I would have frittered my afternoons away). I hated it. And I used to curse him under my breath the whole way home after practices. At the time, hockey and golf were my main sports.
But over time, that changed. In college, I became disillusioned with both hockey and golf, but began to run again (non-competitively) mostly as a way of staying in shape and because I enjoyed the freedom of it. I got inspired by Bill Rodgers and decided to run the Boston Marathon (on about 40 miles per week) during my senior year (1978). Yes, I admit it, I was a "bandit." I wound up doing a lot of walking and finished in 3:32. I was chagrined and was determined to do better. I set breaking 3 hours as my goal.
I have basically been running ever since. And I have enjoyed it more and more, and feel as if have gotten more and more out of it, as the years have gone by. I am now running more than I ever have (about 75 miles a week). I still haven't broken three hours, but I think that, at the age of 46, I have a reasonable shot of pulling it off at Boston this April (I ran 3:02 in Portland this past fall). But that's not even the important thing anymore. What's important is getting out there with my running buddies every day, enjoying both the banter and the feeling of a good run in the woods. And like I said, I can't imagine not running on a daily basis. I'd do it even if I had to run every run alone.
Anyway, my father passed away two years ago and I never did really thank him enough for forcing me to sign up for cross country. I wish I had.