Do writers ask this question of themselves often? A "non-writer" might imagine the answer was part of our writer's business plan before we even started, while literary critics and academics might find themselves out a job if they could no longer surmise ad infinitum about why a particular writer writes. Yet I think we writers hardly ever ask ourselves this question.  We write because we have to. Pondering why we write seems besides the point.  Until, perhaps, we hit that black zone, the scary place we find ourselves when our writing scale becomes alarmingly unbalanced, tipping crazily under the weight of too many rejections and too few successes. 

So why do writers write?  I don't think I've ever read a clearer more concise discussion of what we writers have to clarify and/or prioritize for ourselves in those dark moments than this exchange between writers/professors Sid Lang and Larry Morgan in William Stegner's Crossing To Safety: 

"Ego enhancement, sure. What else? Psychological imbalance? Neurosis? Trauma? And if trauma, how far can trauma go before it stops being stimulating and becomes destructive? Academic pressures to publish, do these mean anything? Not much, we agree. How about the reforming impulse, a passion for social justice? Are writers reporters, prophets, crazies, entertainers, preachers, judges, what? Who appoints them as mouthpieces? If they appoint themselves, as they clearly do, how valid is the commission? If Time alone makes masterpieces...then great writing is just trial and error tested by time, and if it’s that, then above all it has to be free, it has to flow from the gift, not from outside pressures. The gift is it’s own justification, and there is no way of telling for sure, short of the appeal to posterity, whether it’s really worth something or whether it’s only the ephemeral expression of a fad or tendency, the articulation of a stereotype...But the fact is, you can tell, don’t I think?"

...I can’t help suggesting that he has overlooked an important inducement, and that outside pressures do count. The libraries are full of authentic masterpieces that were written for money. Grub Street turns out good things almost as often as Parnassus. For if a writer is hard up enough, if he’s far down enough (down where I have been and am rising from, I am really saying,) he can’t afford self-doubt and he can’t let other people’s opinion, even a father’s keep him from writing.