“Friendships don’t just happen,” says William Rawlins, a professor of interpersonal communication at Ohio University. “They don’t drop from the sky.” Like any relationship, friendships take effort and work. But they’re often the last to receive that effort after people expend their energy on work, family, and romance. And [as I’ve written before](https://www.theatlantic.com/health/archive/2015/10/how-friendships-change-over-time-in-adulthood/411466/), as time goes on, friendships often face more hurdles to intimacy than other close relationships. As people hurtle toward the peak busyness of middle age, friends—who are usually a lower priority than partners, parents, and children—tend to fall by the wayside. Our increasingly mobile world also strains friendship. In one study that [longitudinally followed](http://onlinelibrary.wiley.com/doi/10.1111/j.1475-6811.2007.00158.x/abstract) best-friend pairs, people moved 5.8 times on average, over 19 years. But it’s not just that people move frequently in the modern era—they also cover more ground than they ever have, historically. The epidemiologist David Bradley once [looked at the “lifetime track”](https://link.springer.com/chapter/10.1007%2F978-3-642-73772-5_1) of four generations of his family. “Lifetime track” is a term zoologists use to describe the entire sum of an animal’s movements from birth to death. Bradley found that his great-grandfather’s entire life took place “in a square of only 40 kilometers.” His grandfather’s lifetime track was about 400 square kilometers; his father’s was about 4000 square kilometers, and his own extended all over the world, for a 40,000-kilometer square.