It's rude to use your housemate's things without permission.
Jim, who lives with his roommate Sam, has work the next day but his car is in the shop.
Jim wants to get to work.
Jim gets up early the next morning and rides the bus to work.
Jim takes Sam's car keys and borrows his car to go to work without asking first.
Jim arrives at work on time and has a productive day.
Jim gets yelled at later that day by Sam, who is furious that he took his car without permission.