You might call this life, this lifestyle, the grandest of delusions - that you can tell the stories, and that someone, somewhere, will be interested enough to listen. And you continue to do it, through storms of troubles, through the deserts of despair, because... because... well, for many of us it's the only road there is.
Why do I write? Because I can't not.
Here's another answer:
Civilisation's greatest single invention is the sentence. In it, we can say anything. That saying, however, is difficult and peculiarly painful. Whether we are writing a novel or a letter to our bank manager, we have the eerie sensation that we are not so much writing as being written, that language in its insidious way is using us as a medium of expression and not vice versa. The struggle of writing is fraught with a specialised form of anguish, the anguish of knowing one will never get it right, that one will always fail, and that all one can hope to do is 'fail better', as Beckett recommends. The pleasure of writing is in the preparation, not the execution, and certainly not in the thing executed. The novelist daily at his desk eats ashes, and if occasionally he encounters a diamond he is likely to break a tooth on it. Money is necessary to pay the dentist's bills.
More answers to the eternal question here