Runner's high[1] is one factor - I would presume that the dopamine from mastery (a new discovery compared to the previously believed dopamine release from compl

Runner's high[1] is one factor - I would presume that the dopamine from mastery (a new discovery compared to the previously believed dopamine release from completed tasks -don't have a citation on me right now) is another factor; the pain reminds you that you are pushing your boundaries and getting better physically and at your discipline. The pain itself is probably awful, but it symbolizes a lot to the athlete. [1]: http://www.nytimes.com/2008/03/27/health/nutrition/27best.html http://www.nytimes.com/2008/03/27/health/nutrition/27best.ht...