In no particular order...
I run to think.
When I run, it's just me. It's my time alone. Alone in nature. Alone with my thoughts. Quiet time. To figure things out, to reflect and ponder. To re-consider priorities. To shuffle the cards, and deal again.
I run to escape.
When I run, I have nothing to weigh me down. Nobody can interrupt me, or stop me, or make me do anything. Me, however, now that's a different story.
I run to prove it.
To prove to myself that I can do what I think I can do. What I've told myself I can do. To prove to others too, but mostly to myself. I run because I need evidence. If I tell myself I can do something, but I never actually do it, how do I know for sure whether I'm full of it or not?
I run for health.
It's great for your heart. It's great for your lungs. It's great to build muscle, and burn fat, and all that great stuff. Blah, blah.
I run to eat.
When I run, especially when I run a longer distance, or a longer amount of time, I don't feel too guilty when I eat something that's typically considered less than "healthy." I'm a big-time foodie, not a particularly picky eater; I'll eat almost anything that's placed in front of me.
WHY DO YOU RUN?