Since her husband died, Betsy spends most days in her flat, watching television and boiling tea for one. She is not alone in her loneliness. More than nine million Brits (15 percent of the population) say they often or always feel lonely, and Great Britain has appointed a minister of loneliness to find out why and how to help.

Some causes of loneliness are well known: We move too often to put down roots. We can take care of ourselves, and we don’t have a reason to reach out. We are separated by technology—each of us immersed in our own flickering…