Willpower seems to be about trading off short-term gains for long term ones. In the short term, doing a particular action may not be very pleasant, but in the long term, you do want to have completed that activity. People's preferences aren't generally stable, and what we "want" in the short term is not always the same thing we will "want" in the long term. We use the same word for both, but it doesn't mean that it is the same thing, or that it is impossible for a short term and a long term desire to conflict.