Some people are very anal about things they own, others aren't. I reckon some are a bit too precious with their stuff, it is there to be used. If I'm not using it, it will just be sitting in the garage anyway. The joy others have from using my stuff far outweighs the possibility of a little bit of damage. My friends/family are the type that would return stuff the same/better than when they got it.
BUT, if it was brand new and I hadn't used it I would tell them to get stuffed!
I would lend mine out to my good mates and family no worries. I trust them to look after it and fix it if they broke it. My trailer is not as expensive as some on here, maybe that makes a difference.
Numerous times My Patrol has been borrowed for a week at a time(I have a work car) to tow a work trailer while their car was getting fixed. Each time it has come back with a full tank of petrol, regardless of how much was in there when they borrowed it, and they wash it!!!
A mate borrowed my trailer and broke the rear light. It came back with 2 new lights!
If you have untrustworthy mates/family, that is very unfortunate for you.