Marriage is a social construct purely for the benefit of children.
In an ideal world, for benefit of kids, perhaps, yes.
It's also an economic construct.
Politically it's also a control mechanism (married couples are less anarchists).
But that's all a bit dismissive of the idea of medium and long term relationships that create desirable benefits -- it's like saying attraction is just a matter of certain chemicals that alter heart rate blahblahblah but quoting the chemistry doesn't get anyone laid. ...and the question was, how to improve or maintain relationships? One wants to have that initial feeling -- or 'honeymoon' sensation as it used to be called -- for as long as, or as often as, possible.
(So we come to the personal construct
within the relationship.)
The thing is, friends can funny, witty, generous, fun-loving and amenable,
you just don't necessarily want to live with them.A person in a relationship expects
something else, the man and the woman. I don't mean unreasonable expectation: merely the expectation on which the sexual/loving relationship is built.
She wants to be told she's sexy.
She wants to be told she's attractive.
That she looks nice in that dress or outfit.
She wants to hear "I love you".
She wants to hear it like it's sincere.
She also wants to see you as the man she wants, takes care of himself, dresses, talks, and shows affection in a manly way. And tells her these things in a deep, masculine-sounding voice, preferably with gentle but confident, non-demanding body language.
That's not stuff you do for friends or the office, even if there
seems to be some overlap in acceptable grooming and being well turned out.
This will often require a conscious effort. And that's ok.
There's an amusing crossover... a couple of rhetoricians call it rhetoric, Ross Jeffries calls it NLP. But whatever you call or don't call it, it means making someone feel good about themselves in your company, what they want, often to get what you want, and it doesn't matter if they spot you doing it because it's also genuine. They may even start doing it in return!
The best of books by rhetoricians I enjoyed on this are Sam Leith's 'You Talkin' to Me?' or the slightly more technical 'Thank you for Arguing' by Jay Heinrichs. A while back since I read them but I think it was Sam Leith's that was the funniest (though it might have been Heinrichs). Gave many examples of routines he used with his wife and also with his kids, and how they gradually pick up on it and start using it back at him. When that happens, both parties win of course.