It *really* bothers me when people use my stuff without asking. Except for my mom. That doesn't bother me as much, because we share each other's clothes/shoes all the time. Its actually comical to me when she meets me somewhere in my clothes.
With roomates, its a different story. I'm an only child so I'm used to having total privacy. I had a roomate who would allow her friends to sit on my bed (and who knows what else when I wasn't there!) and it would creep me out to no end. I came back from home one Sunday night to find a strange brownish stain on my comforter. Ew!
I just hate it when people use things they KNOW aren' theirs. My roomates for this year and I agreed to bring our own pots/pans/dishes to the apartment. Well, somehow, it became open season on my cookware. I asked them very nicely to not use it, but they never stopped. And because things were so wierd when I moved out, I haven't gotten my kitchen stuff yet...I hope they haven't trashed it...beyotches...
Thank goodness for private rooms and single apartments!