I'd have to agree with most of what Chanak says. But what keeps me at it isn't really just stubborness. It's the great feeling I sometimes get when I've finished something I'm really proud of, that feeling of accomplishment that tells me I've done something worthwhile. That's what keeps me coming back. For me, the hardest part about writing is getting started. Each and every time I sit down at the keyboard, the struggle begins anew.
Though I might point out that I prefer writing with others. It helps break it up into manageable pieces, and we draw inspiration from each other that helps keep the words flowing.
