Good reply.
As far as myself goes.. mine differs. A large part of the time, my motivation is from Anger, and Hatred towards myself.. Towards my body, and what it is now.. and was in the past. Other times, its strive to be better, out of happiness.. Sometimes, I'm just so happy (thinking about my girl) that I can get that last rep out just thinking about her. Sometimes its also hatred towards someone else.. out of spite, like going "ha, make fun of me now.. just wait.. see what I become".