If you do borrow something, always return it promptly when you're done, and always thank your roommate. Sometimes it's OK to lose. Don't try to win every argument, disagreement, and game of checkers. Let your roommate win now and then. When your roommate gets in a fight with his best friend, girlfriend, lover, mother, or whomever, don't take sides.
Computers are actually fairly private items.
People keep all sorts of sensitive data on their computers. Roommates who try to share a single computer sometimes end up feeling that their privacy has been invaded. Even if you trust your roommate not to snoop or steal, it' s too easy for conflicts to arise. Maybe your roommate will get hooked on the Internet and be forever browsing and chatting when you need your computer to write term papers. Unlikely perhaps, but there are enough possible conflict scenarios to make sharing a computer a bad idea.