If that's a rule then I've got plenty of practice living alone

Not a rule, but as others have mentioned it's good to learn how to be comfortable with your own company. That way you can be more discerning about who to have in your life and why -- less likely to gravitate towards people who aren't good for us just to fill a need.

It's something people tell themselves to feel better about being single.

I am fine living alone I just love it. I also think there are no rules, live as you see fit. Xoxo ?

I’ve wondered this too. While I do agree that we must love ourselves first before loving others, it’s not like a literal prerequisite to be single first.

rules rules rules ... thats too many rules man

I can only speak for one that has lived alone after a 40 year old marriage is it certainly forces a new perspective and discovery of the world around you. You truly realize that at this point, it's a sink or swim life. Every decision is all yours. But on the plus side, you have a more diverse array of options.

When I lived alone I began to discover myself, what makes me tick. I learned my capabilities and my shortcomings, I began to be able to really critically think without the interruptions from others or the constant drama that distracted me.
I have always valued my "me time", if I don't get enough of it I become irritable and short tempered.
Living alone has made living with others more comfortable and enjoyable, even during the rough patches we all have.

Man is a social animal and therefore should be happiest when in company. I think there is some truth in the statement though. You have to be able to enjoy your own company before being comfortable in a social group.

It's a prophylactic against codependency and part of some psychological theories, although Iv never heard it stated as you wrote it. Basically its the idea that happiness comes from within and you cannot be MADE happy by someone else.

Nobody that I can think of. One can be happy alone and happy living with others. I think what this is pertaining to is that after you had a break in a relationship it is good to learn to be happy living by your self as happiness should come from you not some other person.

I want aware it was a rule, just something some people say. Seems like something more said about women. I assume it came out of women having more rights and choosing their own destinies. Girls ussually used to stay at home until they got married. It makes sense not to get married before you have a chance to live outside of the direct influence of authority so you can form some opinions about the world and have a better idea of what you want in life. Also a good idea not to get locked into another relationship without clearing your head a bit after a divorce or big break up. When living with someone it is sometimes hard to separate who you are as a person outside of that relationship.