A combination of factors for me.
I got into the scene by way of comics. Still poses came later, followed by modeling work, and finally animations.
Major motivators for me getting into the scene were puberty (what can I say? Boys will be boys, as they say, :D), a very active imagination, and being fed up with stories in porn. So I decided to make my own stories.
What kept me in the scene is, as Rastifan mentioned, an appreciate of the female body. The human body in general, really - both men and women have, from a purely medical and and kinematic stand-point, absolutely beautiful bodies. That being said, I do revere the female body far more than the male body.
Beyond those points, boredom and knowing that the work that I do is appreciated by others is what keeps me going.