Here’s Paul Auster’s answer.  What’s yours?

Paul Auster said: “I don’t know why I do what I do. If I did know, I  probably wouldn’t feel the need to do it. … Surely it is an odd way to spend  your life — sitting alone in a room with a pen in your hand, hour after hour, day  after day, year after year, struggling to put words on pieces of paper in order  to give birth to what does not exist — except in your head. Why on earth would  anyone want to do such a thing? The only answer I have ever been able to come  up with is: because you have to, because you have no choice.”