Things change dramatically when you add another person into the house. Let's say you get married, so now you have a spouse living there too. You can't just march into the washroom at any given point; you need to check first to make sure your spouse isn't in there!
If you have two responsible adults living in a house, generally you can be reasonably lax about security — you know that the other adult will respect your space, won't try to set the kitchen on fire (deliberately!), and so on.
Now, throw a few kids into the mix and suddenly things get a lot more interesting.