Books That are Not Returned

Why do people think they don't have to return a book they have borrowed? I love my books and I'm willing to share but I do want them back. Thanks. -LMC

I lent you a book but I never got it back
I've kept my mouth shut and cut you lots of slack
I should have charged late fees
Or held on to your keys
I thought you were gracious, but you're really a sad sack.