To me it depends a lot on the item and the relationship
. We've lent money to my parents-in-law. My FIL, an accountant, drew up a loan agreement, with terms and interest and such-forth, and kept to it.
If we were asked for money by my BIL/SIL, though, we'd probably consider it a gift and not expect it returned, and decide if we'd go for it.
I had a history teacher that I loaned a historical fiction book to, one about WWII. She never returned it, even when I shyly asked at the end of the year. I thought she was awesome, but felt just a little resentful every time I looked at my bookshelf where volume 2 sat, because now volume 1 was gone forever. My mother later told me that she really resented that teacher as well, because she asked for pictures of our ancestors for a history project, and mom gave her a rare picture of my great-grandmother, who immigrated from eastern Europe, and the teacher lost them all.
We have two trailers, one smaller (good for hauling furniture, etc.) and one very large (good for making state-to-state moves, etc.), and he's loaned both to trustworthy friends who returned them in the same condition they were lent out.
With books, I'm terrible at remembering that I even loaned them to begin with, usually. I had a friend I worked with who borrowed two books and gave them back months and months later, and I was very glad because I'd forgotten that I lent them and would never have known where they were. I mostly remember the book mentioned previously because it had been volume 1 of a 2-volume set (I eventually got rid of volume 2, as it was useless without volume 1).
My parents and I lend stuff back and forth all the time, anything from DVDs to coolers. To a certain extent there's just a fluidity of belongings, and we won't always remember who a specific item belongs to, and whoever needs it will just ask for it. Usually this is fine, but I do sometimes get a bit annoyed when the kids' clothes get mixed up. See, my mom has clothes for them, so sometimes they'll arrive in clothes from my house, they'll return in clothes from her house, so now a set of my clothes are at her house and a set of hers at mine. In a recent case, she asked a day before the kids were due to go to her place for a visit if I could send a particular item of clothing with them. With four small children, there's a constant mountain of laundry to be done, and if the item isn't clean and in a drawer (it isn't), there's no way I can attempt to find it and have it laundered and ready to go by the next day. For this reason, if I need an article of clothing for something specific (like the 4th of July dresses I bought them), I make sure they don't wear it for over a week beforehand so I can make sure it's laundered and ready.