While some species of animal tend to live as solitary individuals whose only contact with other members of the same species is to fight for resources or to mate, many other animals exhibit social behavior, and live in small, moderate or large groups. Fish live in schools, birds in flocks, wolves in packs, and humans in societies. The reasons are much the same - their respective environments provide economies of scale.
A lone bird would likely fall prey to a predator very quickly. A bird flying in a flock has a multitude of other birds to alert them of impending danger.
A lone wolf without a pack could be picked off easily. But the power bloc of the pack offers protection.
A lone human will struggle just to survive, but by trading with a large number of others offers economies of scale.
In practice, modern human belong to many groups - the smallest being a marriage.