Some people often feel that the drawbacks of social interaction outweigh the benefits. Some other people feel the opposite. At one point both choose to believe that social interactions are or are not of high value, and through daily social friction it soon becomes a permanent pattern of behaviour in their lives. Usually all this happens unconsciously in early life, but as time goes on it becomes conscious why they choose this or that. So i believe in most cases it's a voluntarily accepted pattern, and it has binary qualities.

By the way, introverted people should not be stereotyped as living with their mothers, they are successful enough not to and, indeed most of them value their independence above all.