Serious reply, my best inference:

John Henry was motivated by pride (he was a champion steel driver in contests with other men), gain (the Captain had offered him $50 and a new suit of clothes if he won), and love (he and the Captain had been close for many years, back in Mississippi, where they both grew up - the Captain treated him like a son).

He didn't intend to die, of course, but he had had a recent heart attack (that's when "Polly Ann drove steel like a man") and from the exertion of the contest a weakened section of the wall of his heart gave way - he suffered a ventricular rupture and bled out over the course of a half hour or so.