Writing is a kind of mental illness. Why people do it should be pondered in medical journals. Only a few people can make any money at it, and those people are not always the best writers. But as mental illnesses go, it's better than most. Oh sure, I hear voices in my head, but they're not usually telling me to kill. Usually, they're trying out a bit of dialogue or a description to see how it sounds, or playing with book ideas to see if they make any sense at all.
People write for a thousand different reasons--therapy, fame and fortune, an itch. People who are real writers never really know why they do it, just that they have to do it or run mad.
There is a great pleasure in writing. Nothing can be more fun for the person with the disease than finding just the right word, just the right metaphor, just the right voice for a piece. It's like what a singer feels when hitting the note dead on, without wavering or faltering. For this reason, most writers that I know love words like they love their children. They collect them and roll them around on their tongues like Godiva chocolate, and say umm that tastes good.
If you ask a writer why they write, they say "because I have to." There is no logic to this; it is one of those things, like love, that gets down into your memories, your values, yourself. My wife tells me that if I ever stop writing she will leave me, not because she wants to, but because I won't be the same person. We writers should be pitied, because we cannot help ourselves.