When you live with people you’re bound to see them naked at some point. Probably accidently, unless you’re some kind of peeping tom.
So, what happens when you see your friend’s genitals? It’s only fair you show them yours as well. Right? Right.
But wait! You forgot that there’s more people living in the house, and while you’re comparing wangers they’ve come into the room and seen yours too. Repeat ad infinitum.
This literally could keep on happening forever. Next, the parents pop around, and then it gets really awkward. Which is a good reason not to give them their own key.