“I trained in internal medicine, and I expected most of my time would be spent on diabetes or heart disease or cancer,” Dr. Vivek Murthy, who was surgeon general of the United States under President Barack Obama, told me. “What I didn’t expect was that so many people I saw would be struggling with loneliness.”

More than one-fifth of adults in both the United States and Britain said in a 2018 survey that they often or always feel lonely. More than half of American adults are unmarried, and researchers have found that even among those who are married, 30 percent of relationships are severely strained. A quarter of Americans now live alone, and as the song says, one is the loneliest number.

One gauge of social isolation, or perhaps selfishness: Murthy says he saw families sometimes drop elderly family members off at a hospital for Thanksgiving or another long weekend in what sounds to me rather like the practice of a family leaving a dog at a kennel when they’re going away. At the hospital, doctors are sometimes the only ones to witness a patient’s death, with no loved ones around.

Loneliness affects physical health in two ways. First, it produces stress hormones that can lead to inflammation and other health problems. Second, people who are alone are less likely to go to doctor appointments, to take medicine or to exercise and eat a healthy diet. We may resent nagging from loved ones, but it can keep us alive.