It's the question that is always asked of the author in the room. Jack Kerouac answered it by saying he wrote because he was bored. Charles Bukowski refused to answer it.
I write because I have nothing better to do.
I write because I am immensely vain and like the sound of my own voice, even in silence on the page or computer screen.
I write because I don't know how to say these things in conversation.
I write because I want to be told how clever and talented I am.
I write because I believe telling one's own truth liberates everybody.
That's all. If I get a reputation or a career out of it, fine. But I'm too lazy and undisciplined to seek them actively.