In mainstream American culture, there is no real sense of community. People prefer to live in isolation in their own cocoon so they can feel protected, so that no one can actually bother them. The whole setup of ones life is geared towards isolation. But in isolation, the mind itself becomes a greater danger and threat, becoming more lonely and depressed. Society is like a wild field where any kind of crops can grow, both positive and negative. The field of conflict and difficulties, where and pain, suffering and negative karma arises, is the result of being in society and in close contact with other people.
But in the case of Sangha, that self-centered mind turns around because there is no escape, no hole you could disappear, and so one’s mind begins to work with the teachings. You feel challenged and the more challenge that comes your way, the better off you become by having the opportunity to work with your mind, and test your limits. By training within the small community of sangha, one becomes inscrutable in one's state of peace and harmony with others and in harmony with the world.