Adult friendships usually do not end in an argument. They end in a calendar. Two people who genuinely like each other stop being in the same places, stop having an easy reason to meet, and eighteen months later the friendship is a set of birthday messages. Nobody decided anything. The structure changed, and the friendship followed it. That is an unfamiliar way to think about closeness, because almost all friendship advice is written as though the problem is emotional: be more vulnerable, be a better listener, reach out more. Those things matter. But they are not usually what broke. What broke is the shape of the week — who you happen to be near, how often you have a reason to see them, whether your time together ends in a way that produces a next time. The useful thing about...