If you were to look under the roofs of American homes at random, it wouldn’t take long to find someone who lives alone. By the Census Bureau’s latest count, there are about 36 million solo dwellers, and together they make up 28 percent of U.S. households. Even though this percentage has been climbing steadily for decades, these people are still living in a society that is tilted against them. In the domains of work, housing, shopping, and health care, much of American life is a little—and in some cases, a lot—easier if you have a partner or live with family...