If you've run long enough, you will inevitably get this question. Personally, I find this question very difficult to answer. Yes, there are standard, canned answers... I run to stay healthy, to lose weight, as a means of relieving stress, I enjoy the outdoors, I enjoy the company of others, yadda, yadda, yadda. I've never liked these answers, though. They seem woefully incomplete. Sure, I like feeling healthy, and I really do love the connectedness with nature when running trails. With three small children, running definitely relieves stress! Still, there's a deeper, more primitive reason I run. It is exceedingly difficult to capture in words, but it feels like a drive of sorts. It feels like this is what I was meant to do.
There's an anthropological theory that suggests humans evolved as long-distance runners (the "running man" theory). This idea is explored in Christopher McDougall's book "Born to Run". The idea- we evolved originally to use a method called "persistence hunting". The basic theory works like this- we would hunt animals by running them to death. Most animals are faster than us, but do not have the physiology to maintain long, slow distance running. Eventually they collapse from exhaustion or hyperthermia. I find the theory to be endlessly fascinating in part because of my social science geekiness, but also because it helps explain why I feel the way I do when I run. It helps explain this unknown drive to run ever-greater distances; to continue to push myself to find the point of failure.
In my last race (the Fallsburg Marathon), I was feeling somewhat fatigued around mile 20. At that point, I had run about 45 miles over two days. As I was going through the laundry list of mental tricks distance runners use to survive the cycle of mental and physical peaks and valleys, I stumbled on perhaps my best trick. I saw a runner in front of me. I imagined they were prey and I was hunting them. Suddenly, the pain subsided. I felt a burst of energy, immediate, intense focus, and a surreal sense of happiness... surely it was the result of a massive release of endorphins and epinephrine. It was as if I were suddenly floating over the trails. Within 30 seconds, I caught that runner. I continued this "game" for the next six miles, ultimately passing about twelve runners. Never before have I finished a race so strongly, even though I had just passed the fifty mile mark over two days of running. Next time you find yourself in the clutches of that low the marathoners like to refer to as "the wall", try thinking of yourself as a hunter stalking and capturing the prey in front of you. Channel your ancestral root! You are doing what we were designed to do.