I'm only guessing that every runner asks themselves at one time or another, "Why do I do this?".
I've been running consistently since I was in 7th grade, and I've noticed as time has gone by that my reasons for running have changed. When I first began running it was something to do, a way to be apart of things. I've noticed that with the middle school kids that I coach, it is 98 percent social for them.
When I got into High School it became more about competition and being the best. I did not like to lose. I trained with the boys because I knew it would make me faster. I ran in all sorts of weather, most days leaving the house with my father saying "you're going out in this?".
When I became a Senior running became my way to pay for college. Then, when I went to college, it became my job.
It wasn't until I was about 26 years old that it changed again. My father died of a massive stroke at the age of 56. His father died of a massive heart attack and the age of 56, and his brother died at the age of 54. This was when I realized, I could never stop running. Somewhere along the line my father went from a strong, athletic man, to an old man with diabetes and bad legs. It scared me to think that at 26 I might be at mid-life. If I take after my father (which I do in most respects) then the clock was ticking!
Now I run mostly for health and sanity. The social aspect that was there when I was in middle school has come back. I like having running buddies, although they are sometimes hard to find. I like going to the hash runs just to hang out with other runners. I like being outside and saying I ran the Boston Marathon in a Nor'easter, or I ran outside when we had 3 feet of snow on the ground.
It has turned out that running has been my constant companion for almost 25 years. Everything changes, that's just life, but running will always be there! At least I hope!