Writing a book is a grueling process. Most people think it would be fun to write a book, but in reality it's a lot of hard work.
Sometimes it IS fun, of course. Sometimes the stars align and your muse sings, and you look up at midnight and you've written two thousand words and discovered new things about your story that are just amazing, and you think "this is what it's all about."
But most of the time you're sitting there, alone, staring at your computer and struggling to get a few sentences out. There are a million other things you could be doing with your time, and that nagging voice inside keeps telling you how awful everything you write is, and why are you still wasting your time with this story anyway?
So why do we do it? Why do we go back to the blank page day after day when we could be watching TV, checking email, spending time with our loved ones, or doing the dishes?
We do it because we have to.
I'm not sure that non-writers can understand the form of insanity that writers suffer. Writers walk around all day with stories in their heads threatening to burst out of them at any moment. They'll be at work, or driving along the freeway, or out to dinner with you, and they may seem normal - but all the while people are fighting, kissing, crying, laughing, and living inside their heads. Yes, all writers are a little bit schizophrenic.
We write because these characters, settings, and stories won't leave us alone. The only way to ease this whirlwind in our minds is to write it all down. And when we finish that first draft, we can sit back and be at peace. For a short while anyway, until the next story fights its way to the forefront, demanding to be written, and the cycle starts anew.
When I wrote my first book I had no idea what I was doing but I had these characters and scenes in my head that were screaming to get out. I didn't worry about character arcs, or story structure, I just spilled scenes out all over the page. The book was horrible, but when I was done those characters were finally quiet, no longer clamoring for attention inside my head, and I was able to start working on something new.
We write because we have to, even when the process is agonizing, even when we think we must be the worst writers in the history of the world, even when the dishes are piling up and our loved ones think we're crazy. For people like me, writing is the only way to stay sane.