Even before you-know-what began, making arrangements with friends often felt needlessly tricky. Diary clashes, commutes, and work (or else lying supine and recovering from work) diminished occasions for quality time. To a certain extent, this is a hard-to-avoid part of growing up: In a 2016 study, researchers at Aalto University and the University of Oxford found that after the age of 25, we begin shedding casual friends to focus on closer ties. But that doesn’t lessen the need to show up for the people who still matter. Many of us look back on the friendships we had in younger years as particularly strong.

Childhood rituals like made-up languages, friendship bracelets or secret codes may seem goofy, but they formalize common bonds in an explicit way. Friendships in adulthood can be h...