Homo sapiens didn't survive long on the African savannas as rugged individualists. Alone, they couldn't scare away the scarier animals and, for the most part, they couldn't catch or kill the tastier ones. But in groups, humans rose to the top of the food chain thousands of years ago and have been passing down their tribe-loving genes ever since.
Customs and practices that ensured the survival of the species were worked out through trial and error and passed from one generation to another. Over time, and with many setbacks, the knowledge accumulated until we hit the critical mass required for modernity.
Indeed, the story of modernity is the story of how we moved away from traditional, non-voluntary, forms of tribalism based on familial, ethnic or even nationalistic lines and toward voluntary forms of tribalism.
The American founding was revolutionary in its embrace of the universality of human rights (even as it fell so short of its own ideals with the institution of slavery). Since then, the West has fought several civil wars to break away from various tribal ideologies, including not just monarchism and imperialism but Nazism (racial tribalism), Communism (economic tribalism) and fascism (national tribalism).
In fits and starts, we've moved toward ever greater voluntarism, which is a fancy way of saying we've moved toward greater individual liberty. According to the American creed, no one, and no thing, is the boss of me unless I agree to it. To a certain extent, that's even true -- at least in theory -- about the government, which is a representative institution created solely by and for the people, who are sovereign.
But the instinctive attraction of tribalism endures. The same drives that once pushed tribes to kill the villagers downriver still reside in us. We've just learned to channel and check them better. Bowling leagues, football franchises, high school rivalries, motorcycle clubs, Goth clubs: you name it, these free associations -- what Edmund Burke called "little platoons" -- satisfy our innate desire to belong to "something larger than ourselves," as so many politicians like to say.
Now, in the context of American politics, I would (and often do) argue that the left has grown confused about all this. They've tried to turn government itself into tribal enterprise of some kind. Democratic politicians tell us "government is just the word we use for those things we do together." "We're all in it together!" has become at once a rationalization and battle cry for larger government and higher taxes.