I don't understand people who rent their things out yet are surprised when folks treat them like a rental rather than borrowing something from a friend. The thing is, when I borrow vehicles or skis, or stay in a friend's house, I don't pay them. I may refill the tank or refrigerator, but otherwise, it's free. Once you demand money from someone, it's just a transaction and no you no longer get the friend treatment / stuff treated with kid gloves. Now I (obviously) don't go out of my way to trash rental things, but I certainly don't treat them with as much care as something I borrowed from a friend.

When I rent cars, I think it's fine to leave minor messes in them; it's implied as part of the rental service they'll vacuum carpets or recycle a stray bottle or two.