Getting better feels good. Being able to go a little farther everyday makes you feel a sense of accomplishment. This feeling might be why people keep running. I know for me, whenever I get a personal record in a race, I feel wonderful.
Another reason that's probably the most popular one is to keep getting in shape. Running makes you become more healthy. No matter how big you are, running will make you lose weight. For many people, this might be the only reason they're running. But it's a great reason. You don't have to race and compete to be a runner. All you have to do is just run and you're one of the crazy runners who put themselves through constant punishment.