Agression and running

Over the years of pushing hard on most runs, I have wondered what drives me? What drives other runners? For me to be on a training run or ride and get in the groove of pushing hard is propelled by agression. Anger makes you make mistakes and pushes you over the limit, agression allows you to tap into something to handle the pain. There is no greater joy than being on a training run with an equally matched partner and dropping the hammer. It does not matter who overcomes the other but, the agression has allowed me to push places I never knew I could. No Zen like state for me of the miles passing, just drop the hammer and let it rip. This how ever does not seem to work for runs over 2 hours. By that point fatigue has bled out all my aggression. I have learned over the years that if I am out front of another runner, I am very hard to catch  but, usually don't provide much of a fight once passed. I have given all I have by that point. What drives you to get out there and exercise or race?