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, surveys show 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.

https://www.theatlantic.com/health/archive/2015/10/how-friendships-change-over-time-in-adulthood/411466