It's not really that they're more comfortable, per se, but that you can fall asleep a lot easier on someone else's bed. And I don't mean after sleeping with the person. Sometimes it can take me hours to fall asleep in my own bed, and I have a pretty comfortable bed - I used to have a waterbed, too, and I experienced the same phenomenon. If I go crash at someone's house and they whip out the bed with the weird stain that you can't decide is coffee, pee, or the mark left by death, I'll still be out like a light. Try it out. Swap beds with your sibling or roommate for a night. Help validate why I take naps on other people's beds.