That's an interesting question. First and foremost, I write because I enjoy writing. If I don't -or can't- write for more than a day or two, I feel it. I miss the physical act of writing, of creating something from nothing more than my own imagination. But, beyond that, I write because I want to make my living doing it. I don't have to be as successful as Stephen King or John Grisham, but I'd like to be able to live a comfortable life as an author. I also write because each of my books is better than the one before it. In my opinion anyway, so I feel like I'm improving as a writer.
Since my days as a songwriter, I've always wanted to be able to support my family doing something creative. I honestly can't think of a better way to make a living.