Because there's this story we really want to read, and nobody else has written it yet.
For me, it's an adventure that has the world-righting clarity of Dorothy Sayers, the homely joy of Essie Summers, the wry brilliance of Lois McMaster Bujold and the sheer sexy goofiness of Dr. Who.
Can I write it? I don't know. But it ought to exist and so I have to at least try.
And try and try and try. Which leads us to the answer to our second question: why do writers write more than one book?
Because we never quite hit what we're aiming at. But we get close enough to lift up our hearts, and so we have to bend the bow, close one eye, focus on the target, and let loose the arrow once more.
Because there's that one story that we really, really, really want to read. And nobody else has written it yet.
Peace of Christ to you,