Human beings are social animals, and the tenor of our social life is one of the most important influences on our mental health. Without positive, durable relationships, both our minds and our bodies fall apart. We begin life dependent for survival on the quality of relationship with our primary caregiver, usually Mom. And the nature of that relationship typically influences all others in our life.
Our survival as a species similarly hinges on our capacity for social living. Most of human history was spent in small groups in which each was dependent on the others for survival, and evidence suggests this is the condition to which we are best adapted.