Steve Pavlina: "First, thinking that you absolutely have to do something is a major reason for procrastination. When you tell yourself that you have to do something, you're implying that you're being forced to do it, so you'll automatically feel a sense of resentment and rebellion. Procrastination kicks in as a defense mechanism to keep you away from this pain. If the task you are putting off has a real deadline, then when the deadline gets very close, the sense of pain associated with the task becomes overridden by the much greater sense of pain if you don't get started immediately."
via Keith Devens, who's putting off reading it.