The reason is simple. By our very nature, people are gregarious. They seek companionship from their fellow beings. The prefer to live in communities. Life is, in the end, a series of relationships.
Solitary confinement is the ultimate prison punishment.
In school, we are part of a community. The Bible tells us husbands are one with wives, and wives with husbands. We belong to teams, organizations, congregations, nations. Minorities search out the security of ethnic identification. Today, more than ever, we live in such a big society.