In the hierarchy of relationships, friendships are at the bottom. Romantic partners, parents, children—all these come first.

This is true in life, and in science, where relationship research tends to focus on couples and families. When Emily Langan, an associate professor of communication at Wheaton College goes to conferences for the International Association of Relationship Researchers, she says, “friendship is the smallest cluster there. Sometimes it’s a panel, if that.”

Friendships are unique relationships because unlike family relationships, we choose to enter into them. And unlike other voluntary bonds, like marriages and romantic relationships, they lack a formal structure. You wouldn’t go months without speaking to or seeing your significant other (hopefully), but you might go that long without contacting a friend.

Still, survey upon survey upon survey shows how important people’s friends are to their happiness. And though friendships tend to change as people age, there is some consistency in what people want from them.

“I’ve listened to someone as young as 14 and someone as old as 100 talk about their close friends, and [there are] three expectations of a close friend that I hear people describing and valuing across the entire life course,” says William Rawlins, the Stocker Professor of Interpersonal Communication at Ohio University. “Somebody to talk to, someone to depend on, and someone to enjoy. These expectations remain the same, but the circumstances under which they’re accomplished change.”

The voluntary nature of friendship makes it subject to life’s whims in a way 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 voluntary nature of friendship makes it subject to life's whims in a way other relationships aren't.

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.

* * *

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, young adults often spend between 10 and 25 hours a week with friends, and the 2014 American Time Use Survey found that people between 20 and 24 years old spent the most time per day socializing on average of any age group.

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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, like 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 professor of communication studies at Texas Christian University, found that participants had moved an average of 5.8 times during that period.

“I think that’s just kind of a part of life in the very mobile and high-level transportation- and communication-technology society that we have,” Ledbetter says. “We don’t think about how that’s damaging the social fabric of our lives.”

We aren’t obligated to our friends the way we are to our romantic partners, our jobs, and our families. We’ll be sad to go, but go we will. This is one of the inherent tensions of friendships, which Rawlins calls “the freedom to be independent and the freedom to be dependent.”

“Where are you situated?” Rawlins asks me, in the course of explaining this tension. Washington, D.C., I tell him.

“Where’d you go to college?”

“Chicago.”

“Okay, so you’re in Chicago, and you have close friends there. You say ‘Ah, I’ve got this great opportunity in Washington…’ and [your friend] goes ‘Julie, you gotta take that!’ [She’s] essentially saying ‘You’re free to go. Go there, do that, but if you need me I’ll be here for you.’”

I wish he wouldn’t use me as an example. It makes me sad.

* * *

As people enter middle age, they tend to have more demands on their time, many of them more pressing than friendship. After all, it’s easier to put off catching up with a friend than it is to skip your kid’s play or an important business trip. The ideal of people’s expectations for friendship is always in tension with the reality of their lives, Rawlins says.

“The real bittersweet aspect is young adulthood begins with all this time for friendship, and friendship just having this exuberant, profound importance for figuring out who you are and what’s next,” Rawlins says. “And you find at the end of young adulthood, now you don’t have time for the very people who helped you make all these decisions.”

The time is poured, largely, into jobs and families. Not everyone gets married or has kids, of course, but even those who stay single are likely to see their friendships affected by others’ couplings. “The largest drop-off in friends in the life course occurs when people get married,” Rawlins says. “And that’s kind of ironic, because at the [wedding], people invite both of their sets of friends, so it’s kind of this last wonderful and dramatic gathering of both people’s friends, but then it drops off.”