Now
there's something noticeable by its presence in a 2012 article - a Louis CK quote.
Anyway the article names various and sundry complicated and small manifestations of this phenomenon, but what's the big picture? Maybe making friends past a certain age is hard because it's unnatural and somewhat irrelevant? Maybe making friends is just another naturally- and reproductively-selected trait that helps with mating and is much less relevant otherwise. Meaning that wanting friends, results in having more friends, which means more hunting/foraging helpers and teachers of same, which makes you more likely to survive long enough to mate, and more likely to find a mate (since all your friends will have siblings etc., and the ones from outside your tribe are particularly valuable statistically/genetically speaking) and successfully pass that friendly tendency on to the next generation.
Or heck maybe wanting friends is an indirect and sublimated sexual urge in and of itself. It's not like homosexuality comes outta nowhere. And/or maybe being with one person is psychologically a surrogate or substitute for (or practice for later) being with your mate, just like owning a dog is to having children.
Anyway, once all the mating stuff is over with, maybe you're immune to it all. I don't have much to go on here, other than 1) my tendency to look to evolution and the Ape for explanations of human behavior, and 2) the fact that I noticed I lost interest in friends as soon as I found a mate I was happy with. And interestingly, being with the previous one, whom I wasn't happy with, didn't put a damper on my social tendencies at all. (But I was younger then too, so there's at least one confounding variable.)
Regardless, I suddenly notice there are large swathes of life that seem to be there only to trick you into mating. (That is, the ones that aren't there to trick you into giving someone money.)
(Some people use the one to accomplish the other.)
(And vice versa.)
But then you reach a point where you're immune to all that, and it's a relief. You can finally get on with it, whatever the "it" is in your life. We flatter ourselves that life is complicated, important and purposeful, but actually it's pretty simple and probably random. Life just wants us to mate & raise kids, and then after that, it could give a shit what we do. (As long as we keep out of the way of other people mating & raising kids, of course.) That semen/those eggs are just your genes heading for the lifeboats of a sinking ship. But that also means once you're freed from "life's plan for you," you can stop giving a shit right back, and finally do whatever you want!