Writing is the best way I know to look inward. It's more fun than therapy, more effective, and it has the additional virtue of being free. In fact, sometimes -- in extraordinary circumstances -- people pay you to do it.
I started writing because I heard a constant babble of voices in my head, loud enough and varied enough to make me wonder whether I had multiple personality disorder. After writing for a few years, I discovered that multiple personality disorder is something to be cherished, to be watered regularly and taken for the occasional walk on the lawn.
Multiple personality disorder is the short cut to characters, and characters, in addition to being indispensable to fiction, are all slivers of the self. They may not be especially pleasant slivers, and it may be disconcerting to know that you're harboring a small crowd of Mr. Hydes and Dr. Mengeles, but there are angels in there too. We all of us contain the bruisers, the bruised, and the healers. We should buy them cupcakes from time to time. It's important to know they're all there.
So writing is one way to circle the mystery of who we are. We bring our warring cloud of inner children to the tips of our fingers and let them do their stuff. And then, sometimes, something very interesting and slightly mysterious happens. They create a story, and that story arrives wrapped in its own world, and that world has its own weather and landscape and rules. And if you nurse it along for an extended period and let the characters have their say and do what they would in the circumstances you've imagined, you have a novel.
A novel, whatever else it may be, is a projection of the person who wrote it. It's been said frequently that a writer can't create a character more intelligent than than the writer is. I'm not sure about that, but there's no question that writers can create characters braver, more cowardly, more evil, more saintly, more almostanything than the writer is -- because the writer as a functioning personality is a carefully assembled presentation of the good/bad/beautiful/ugly/wise/immature inner voices in his or her skull. Part of growing up is to learn to manage our conflicting impulses, to organize them, like a good photographer faced with a motley crowd and somehow creating a relatively attractive group shot. Sooner or later, we begin to believe (at times, anyway) that that carefully assembled jigsaw puzzle is really who we are. Writing lets us pick that apart and speak to each of those little imps and angels individually and let them stretch their legs.
I've been horrified by what some of my characters do, while others have (embarrassing confession ahead) moved me to tears with their goodness. I have rarely moved myself to tears with my own goodness, but it tells me something when I create a world that contains such a character. It's reassuring. And for some reason (maybe self-protection) it doesn't negate that reassurance that I also created the Madame Wings and Captain Teeth who move my stories along with their badness.
So far, I've said nothing about writing well, nothing about art or even competence. I write as well as I can because it gives me pleasure, and I'd do it even if I wrote much less well than I do. I think that writing well is the last thing writers should think about. The first joy is letting the story take shape, living through the characters and exploring the world they inhabit. The second thing is bringing it to some sort of completion that's organic and unforced. If you do all of that -- and if you don't censor or bully the slivers of you that appear on the page -- you'll produce something interesting. If you write it simply, trying to keep the prose out of the way so the pages are windows through which the reader sees the action, you'll have a working first draft. Then, if you want to, you can worry about making it better.
So the real reason I write is that I can't think of an answer to the question, Why shouldn't I write?