I agree with James. I absolutely can't stand when people take my stuff without asking, or when they enter my room without asking. In fact, I just moved out of a house where one of my roommates went into my room within the first two weeks of my living there. That totally made me not trust her right off the bat. But then one night I had a bottle of wine sitting out (which I paid $20 for... not really expensive or anything, but she always buys the $6 stuff) and she drank all of it.
Sorry for my little side story
But I don't think you're overreacting... in fact, I'm glad to know I'M not the only one who inwardly freaks out about this stuff!