"Even in a close and strong relationship like a marriage, a certain amount of blindness may help. While the idea remains controversial, some researchers argue for the value of so-called positive illusions, the rosy image that some people hold, despite the available evidence, about their romantic partners. The psychologist Sandra Murray at the University of Buffalo has found that couples that maintained positive illusions about each other tended to be happier than those that didn’t. Something similar may be at work in close friendships. And, according to Dunning, a slightly different form of social illusion may also arise. People naturally seek out those they see as most like them, and a falsely inflated sense of similarity may only further cement friendships. In other words, one of the nicest things a friend can do is let us misunderstand them just a little. “If you don’t know everything about someone else, you still enjoy the time you spend with each other,” says Delia Baldassarri, a sociologist and assistant professor at Princeton who has studied people’s perceptions of their friends’ political attitudes. “In certain ways, you may even enjoy it more."

What you don’t know about your friends - The Boston Globe