I'm usually pretty easy about loaning things, but I've gotten more reluctant lately. I loaned my copy of "The Night Listener" by Armistead Maupin (purchased on the day it came out, so *sort* of a "first edition") - she misplaced it during a move and her lack of concern about it started making me more reluctant to loan.
I'm in a similar situation about lending my car to my roommate (currently without a vehicle). He's of the same "you're not using it at the moment" mindset. I'm always uncomfortable when he wants to take it to the bar (usually after I've gone to sleep), I worry that he's going to get ticketed and the whole hassle ensuing from that. I loaned a car to a friend for an extended period of time, he got pulled over and ticketed while driving it, and until that was settled the city confiscated the license plate. I don't want to go through that again, especially now that it's my only car (having a spare to loan was an unusual situation for me). The roommate's much more responsible than the guy who got ticketed, but still.