Back in the 1980s, the Oxford psychologists Michael Argyle and Monika Henderson wrote a seminal paper titled “The Rules of Friendship.” Its six takeaways are obvious, but what the hell, they’re worth restating: In the most stable friendships, people tend to stand up for each other in each other’s absence; trust and confide in each other; support each other emotionally; offer help if it’s required; try to make each other happy; and keep each other up-to-date on positive life developments.