Lending out books destroyed my faith in people.
Since my mother retired, she devours books; unlike me, she does not like re-reading them unless she's desperate (me, I will have a character or a storyline from a particular book pop into my head and have to go and rediscover it--that's why I read the HP series every summer, I love to go back and visit those characters in their world). It's very hard to refuse to lend her books to read, neither one of us can afford to keep buying new ones at the rate she reads them, but oh, she returns them with cracked spines and it drives me insane. A few years ago, I couldn't help it, I pointed it out to her and asked her to try to not do that, so now they come back strangely twisted from her effort not to bend them too far back. Can't win, but what can you do? She's 79, so someday I'll be wishing she was cracking the spines of my new books.
(and yes, I have tried suggesting that she get a library card)