We all crave companionship. So being together with someone fills us with the happy feelings of sharing. Now if that someone is a "steady" someone, then you get to tell him/her "hey, remember when..." and share more laughs together because you'll have same memories. I find this precious.
Now society and religion invented "marriage". Due to the fact that women were considered weaker, thus always needing to be taken care of -no job- and because they had the role of the baby-making machines....there had to be a contract that would "secure" them financially and make them also "the property"of their husbands. Now religion had to set some rules, boundaries and set this idea of marriage....so that people could narrow down

the list of people they could have sex with....hahaha! Otherwise things would have gone crazy....hahaha!
Anyway, I know about the loneliness you are talking about. I feel it too. I don't miss the idea of "marriage". But I'm longing for companionship and for kids. I don't think society has anything to do with this feeling. I think it's biological. I need a companion and I'm curious to meet my kids! Society simply shuts up when they see me have all this AFTER wearing the white dress, etc.....if a party is all they need to shut up, sure, I'll do it!! hahaha! A 'copy-paste' life so that everybody can relax knowing that I'm not doing anything different from them.