I saw this on the sound of science blog and think it rings true :) LOL
"Scrapping is almost like puking. I know, it sounds absurd, but hear me out. Once you feel that urge, your muscles take over and just start contracting, whether you want them to or not. It seems there is nothing that can stop it untill it is out. And that is exactally how I feel about creating. Like it just spews out of me at random. Busting out page after page...like a madwoman. And then it's gone, as quickly as it came. Nothing else left inside. But the warm fuzzy glow of letting it all out. It really is therapy."