An article I read a while ago talked about a couple living in separate apartments in the same building and why this works out for them. They met relatively late in life, they were both single with incompatible lifestyles. She has an apartment full of memorabilia and small items, she is neat, she cooks, she keeps pets, and has a deep feeling for her home. He likes simple, clean designs, large, functional items, doesn't cook, doesn't like pets, and is not particularly tidy. They waited for an opportunity to get closer, which presented itself in the form of an apartment for sale right above hers. And since they don't want children, this arrangement is absolutely perfect for them. They prevented the petite quarrels over toothpaste, clothes on the floor and such. If you are in a similar situation and can afford it, it's absolutely genial.
Young adults living with their parents is a new trend fueled by insane housing prices. Parents of such kids are tempted to continue and educate them and this causes continuous friction. It's hard to let go and not try perfecting them "for their own sake". Here is a small tip I discovered myself and I'm sharing it with the world. I used to be mad at [expensive] clothes thrown on the floor and yell, until one day I got so mad, I decided to do something about it. I can't not care, but I can take one minute and hang the clothes on hangers. Decided and acted on. No more yelling, no friction. I told him it's really easy and preferable for me to do this and not be mad anymore. I was ready to go on with this arrangement, but after a coupe of days, he felt uneasy and started doing it himself. "Let me know if you want me to come and hang your clothes for you". "No need, I'll do it, mom". Shows that if you are serious about something, it works.