Although it is not completely clear how they do it, social support networks have been shown to be an important determinant of both physical and mental health.
The most likely connection has to do with the effects of social support networks on stress. People who have social support networks have many ways to relieve stresses that arise in their lives. They have people who will support them in hard times and they will not, therefore, have to bear all of the stress themselves. This can help them remain healthy where people who remain stressed experience poorer health.