My psych major friend is working on a theory that procrastinators have naturally low levels of endorphins, so they subconsciously use the stress of procrastination to raise those levels.
I'd heard that procrastinators were perfectionists, and so all of their tasks loom like enormous projects because they envision all the thousands of tiny details they'd have to get just right, while normal people just have this rough idea of the task that doesn't look so hard. So then the procrastinators put it off because they just don't feel like taking on an enormous project. Which is funny, because in saying this I'm reminded of pg's essay that talked about starting with easy small projects rather than big earthshattering ones, because you'll never actually finish those.
I thought that was a pretty fascinating theory.