There's a machine at the gym where you lie on your stomach and lift a weighted lever by your ankles until your feet are near your buttocks. It works a major muscle in the backs of your legs, which I think is called the biceps femoris. A funny thing happens when I do that exercise: I feel a distinct euphoria. Call it a high if you will. No other weight machine gives me that same feeling.
Now combine this with my observation that people who are addicted to apparently boring sports such as swimming or running tend to have extra large biceps femoris and you have my newest hypothesis: Could the euphoric feeling that comes from working that particular muscle be the reason some people need to run ten miles a day?
Obviously people who exercise a lot will have larger muscles than people who don't. And it's no big secret that exercise makes people feel better in a number of ways. All I'm adding to the mix is the thought that perhaps that particular muscle is more responsible for athletic addiction than others.
I think you'd find that addictive sports are either intellectually challenging (golf, baseball, etc.), or they work the biceps femoris muscle, such as running, biking, and swimming.
I recently joined an indoor soccer team. It exercises the biceps femoris muscles, but it also requires a lot of thinking to make the right passes and plays. Indoor soccer is a fast game, mentally and physically, because you're never far from the ball. My theory predicts that this would be more addictive than most sports. I can attest that it is in fact insanely addicting. I've never experienced anything quite like it. I can see why soccer is the most popular sport in the solar system.
I wonder if the world would be a nicer place if everyone exercised their biceps femoris muscles and experienced its euphoria. A sentence you never hear on the news is "He was a marathon runner and a serial killer."