I think it's because I know that if I don't do them then they won't get magically done. :P Either I do them or it doesn't happen. That's pretty good incentive to get up and get them done. Plus being alone in the house there's not a whole lot else I could be doing. Add to that I don't have to worry about someone peeking over my shoulder and telling me I'm doing it wrong. :P That's very off-putting. Aaaand I feel a tinge of resentment when I'm doing chores and my housemates aren't.
I'm not complaining. I know I don't pull my weight most of the time. It's just refreshing to actually feel motivated for a change.