In their natural habitats, social species organize into characteristic groups. Gazelles form herds, wolves form packs, and ants form colonies. Humans, in the same way, form tribes.
Of course, we’re pretty far removed from our natural habitat these days. But tribes are a large and fundamental part of our evolutionary heritage, and they have a corresponding influence on our mental and social lives. Organizing ourselves into tribes is one of the ways we manufacture normalcy. It helps our paleolithic minds perceive and act, more or less sensibly, in an increasingly complex modern world.
Humans also form kingdoms, nations, states, and civilizations, but those units of organizations aren’t as fundamental to our psychology.