Not that I would know from experience, but here are my reasonings: You devote every minute of the day to it. You worry about it all the moments you aren't with it. You burp it to make sure everything is flowing together smoothly. You change out old, used diapers (also known as "chapters") for new and improved ones. It goes through stages. Sometimes it will cooperate with you, letting you write when it's good for you. Sometimes it's stubborn. And sometimes, always in the rare occasions when you aren't thinking about it - say, when you're sleeping - it wakes you up, demanding attention.
That's the stage my novel is in right now. The wake-you-up-in-the-middle-of-the-night-to-write-a-chapter stage. It cries in my ears when I try to fall asleep, accusing me of negligence. And I know that if I don't answer its call, I will find it dead in the morning, panting on the floor, looking at me with big, black and white eyes asking, Why? Even if I am selfish enough to fall back asleep, I can't stand guilt, so I'll wake back up again either way.
Or maybe, if I've had a really bad week at work and can't muster the strength to get up, I'll forget exactly what the story had been telling me, and that Oh So Urgent call that I ignored will never make it into my novel.
Yeah. Like I'd let that happen.