“Plan an appropriate time to talk before the departure,” recommends one web page entitled “How to Say Goodbye.” The cartoons that accompany it are as stilted and formulaic as each piece of advice. “Talk about all the good times you’ve had,” it urges. “When it’s time to leave, make it brief and sincere.” Implicit in its offerings: To say goodbye can be painful or terrifying. Absence, even necessary absence, can create guilt, and yet, it’s also essential in order to experience the world. “That’s why I went,” says one character in Susan Sontag’s Unguided Tour: “To say goodbye. Whenever I travel, it’s always to say goodbye.”

A wave, a bow, a block on the phone. There are different ways of saying goodbye. Some are necessarily worse than others: a lover’s...