The voluntary nature of friendship makes it subject to life’s whims in a way that more formal relationships aren’t. In adulthood, as people grow up and go away, friendships are the relationships most likely to take a hit. You’re stuck with your family, and you’ll prioritize your spouse. But where once you could run over to Jonny’s house at a moment’s notice and see if he could come out to play, now you have to ask Jonny if he has a couple hours to get a drink in two weeks.

The beautiful, special thing about friendship, that friends are friends because they want to be, that they choose each other, is “a double agent,” Langan says, “because I can choose to get in, and I can choose to get out.”

Throughout life, from grade school to the retirement home, friendship continues to confer health benefits, both mental and physical. But as life accelerates, people’s priorities and responsibilities shift, and friendships are affected, for better, or often, sadly, for worse.

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The saga of adult friendship starts off well enough. “I think young adulthood is the golden age for forming friendships,” Rawlins says. “Especially for people who have the privilege and the blessing of being able to go to college.”

During young adulthood, friendships become more complex and meaningful. In childhood, friends are mostly other kids who are fun to play with; in adolescence, there’s a lot more self-disclosure and support between friends, but adolescents are still discovering their identity, and learning what it means to be intimate. Their friendships help them do that.

But “in adolescence, people have a really tractable self,” Rawlins says. “They’ll change.” How many band T-shirts from Hot Topic end up sadly crumpled at the bottom of dresser drawers because the owners’ friends said the band was lame? The world may never know. By young adulthood, people are usually a little more secure in themselves, more likely to seek out friends who share their values on the important things, and let the little things be.

To go along with their newly sophisticated approach to friendship, young adults also have time to devote to their friends. According to the Encyclopedia of Human Relationships, many young adults spend 10 to 25 hours a week with friends, and the 2014 American Time Use Survey found that people aged 20 to 24 spent the most time per day socializing on average of any age group.

College is an environment that facilitates this, with keggers and close quarters, but even young adults who don’t go to college are less likely to have some of the responsibilities that can take away from time with friends, such as marriage, or caring for children or older parents.

Friendship networks are naturally denser, too, in youth, when most of the people you meet go to your school or live in your town. As people move for school, work, and family, networks spread out. Moving out of town for college gives some people their first taste of this distancing. In a longitudinal study that followed pairs of best friends over 19 years, a team led by Andrew Ledbetter, an associate communications-studies professor at Texas Christian University, found that participants had moved an average of 5.8 times during that period.