Natural societies are small groups of people, held together by kin selection and by trading favors with other group members. If the group grows above a certain size, these effects get diluted, and the group splits up.
Since larger groups are more advantageous for defense and for aggression, methods have been invented to increase cohesion of such groups. Those methods include religion, culture, racism, chauvinism, all kinds of scare tactics, alone or in combination. It is hard to find a society that is held together by love.
