Some people get high off due to the hypothalamus that releases endorphins to provide second wind that keeps a runner going and hence this causes sickness. Endorphins are produced during exercise, excitement, pain, consumption of spicy food, love and orgasm. Read more from http://en.wikipedia.org/wiki/Endorphin.
Some people get what is called a high from running or other physical exercise because the body releases endorphins which are natural pain killers. Some people get sick because of over exertion, dehydration or malnourishment.