I think about this topic so often, I'm surprised I haven't actually written a post about it. This is a good question. Why do I write? Why do I skip sleep, sacrifice social interaction, food, just about every basic need a person could have to write? There is only one answer: Because I have to. Writing for me is an art. Artists are passionate to create what they see, feel or hear and then share what they see, feel or hear with others. I've always loved making people laugh and cry. That's why I gravitated toward theatre in my youth. Writing is an extension of that. And it's a cheap hobby.
The other reason is for me. I use my novels to work out my demons (I have many), express feelings I may not want to deal with in any other form, maybe even discover feelings I didn't even know I had. Writing helps me release those feelings, deal with them and move on. It's pretty cheap therapy.