Honestly? Usually what motivates me to write is all the nice comments you guys leave. It's a lot of fun to check my email during my down-time and see what people have said about my stories. It's a small narcissistic pleasure every time I get a compliment.

That said, there's not really any sexual motivation behind my stories. I'm definitely kinky, and enjoy spanking the occasional beautiful girl, but when I'm writing it's usually more about the relationships between the characters, and the internal struggle of the naughty child. There's also a definite element of my own childhood in some of the stories. I'm sure some people get off to them, and I don't object particularly, but I've never really thought of what I'm doing as writing pornography.
Also, I don't really feel any need to write unless I have some new idea or theme I want to explore. I don't usually write just for the sake of writing.