I had a quick, knee-jerk reaction to your question, along the lines of "because I like it." But then I actually stopped and really thought about that question, in the context of a conversation I had yesterday.
A guy I've seen often at the gym approached me in the locker room and asked me if I was training for something in particular. When I said that I was training for a half-marathon next month, he said that he figured it was something like that, because (his words) "you seem a little bit more committed to your workouts than most of the other folks you see around here." We got to talking about running, and he mentioned how he really only runs now because he feels he HAS to, not because he necessarily likes to. He said he hasn't felt that "runner's high" in a long time, and wonders if he'll ever get that back. Naturally, I started extolling the benefits of barefoot running, inasmuch as it has so renewed my own running life.
Running started as a means to an end, way back in my youth: to get into shape and lose some weight. It became something of an obsession, only to be supplanted by other obsessions over the years. But I always, always, came back to running, no matter how long an absence. There's something in the primal act of moving swiftly through space under one's own power, without the encumbrance of machine or motivation, that just feels right, and doing so barefoot only serves to heighten that feeling. The health benefits of running are obvious, but it doesn't really take all that long to attain them. And then you start doing things like racing, or running long, long distances, or both, and at that point, the real question of why you're running is a fair one to ask. For me, running truly is an expression of who I am -- more comfortable alone than in groups, more inclined toward introspection than outward expression, more willing to spend long hours with only my thoughts and my will, not so much testing either, but simply examining their interplay. I want to run, and I CAN run, so I do. I don't have illusions that it will make me death-proof, or that I'll set any sort of record. It doesn't make me a better person, and it doesn't fix anything that's wrong with the world. It just feels as much a part of me as breathing. So, I run.