Other runners have this experience, also. In a sense, some runners run a race against themselves. If you give in to the personality that wants you to quit, then you've lost your race. You've lost the race not because you didn't beat the winner, but because you didn't beat yourself. People begin running to compete with others or against themselves, to enjoy the outdoors, to experience the runner's high or to stay fit.
Runners run for different reasons but all runners gain the same benefits from the sport.
Running, besides being enjoyable from the scenery and fresh air, if properly done, keeps bones healthy, blood circulating properly, the heart healthy, weight under control and the mind sharp. With all the benefits of running it's understandable why it's one of the world's greatest sports.