The reason it's such a favor is because of how inherently unfair it is to her. Mass-wise I'm roughly twice the size of my wife, but my clothes seem at least three times as large as hers. How long would it take her to create a load of laundry with just her clothes? They just seem so tiny!
Anyway, my wife is out of town and I miss her, and not because I have to do the laundry. I'm not going to pretend it's not a nice bonus, but I would be just as happily married to her even if she insisted that I do it (though I might politely suggest that the shirt she wore over that other shirt for just a couple hours could probably be worn again before it needs to be washed).
I know this has more to do with me than with her, and I mean this is in the most complimentary of senses, but I just don't seem to get anything done when my wife is around. She is the best possible distraction. And while I don't doubt at all that the preceding two sentences are true, it's kind of a personal letdown when she goes away and I still fail to get anything done. Unless you count this. But I don't.