Why Is It Hard to Make Friends Over 30? (2012)

(...) In studies of peer groups, [a psychology professor] observed that people tended to interact with fewer people as they moved toward midlife, but that they grew closer to the friends they already had.

Basically, she suggests, this is because people have an internal alarm clock that goes off at big life events, like turning 30. It reminds them that time horizons are shrinking, so it is a point to pull back on exploration and concentrate on the here and now. “You tend to focus on what is most emotionally important to you,” she said, “so you’re not interested in going to that cocktail party, you’re interested in spending time with your kids.”

As external conditions change, it becomes tougher to meet the three conditions that sociologists since the 1950s have considered crucial to making close friends: proximity; repeated, unplanned interactions; and a setting that encourages people to let their guard down and confide in each other, said [a professor of sociology and gerontology]. This is why so many people meet their lifelong friends in college, she added. (...)

https://www.nytimes.com/2012/07/15/fashion/the-challenge-of-making-friends-as-an-adult.html