Yes, I push myself to silly lengths sometimes and I suppose one must answer why - it is all part of that "seeing how far you can go" thingy. Both distance and time. It is very rewarding afterwards to have done well, and very disheartening not to have done well. This can help motivation. The latter can be negative however, one can feel jolly disappointed in "only" achieveing a 4:30 marathon, but that;'s silly. If you know you trained well and ran well, you should be jolly pleased. Beat a PB - over the moon, no matter where you came in the "race" - after all, mthe majority of us are racing ourselves and our dreams, not the other people.
Ha! that last sentence is quite cute.