There's absolutely nothing wrong with being a close-knit group of people. In fact, emotional closeness is a basic human need. It only becomes toxic when that group defines itself by its exclusivity. It becomes highly toxic when the in-group members disparage outsiders and judge them by different standards; or when "heretical" thought becomes a cause for exclusion; or when anyone seeking to join must "pay their dues" to be included, up to the point of hazing like a frat or sorority.

People need to feel acceptance and belonging--two things so badly missing in society. But tribalism and cliquishness can quickly get ugly and become forces of exclusion for anyone not lucky enough to be an insider. Any time there is an "us," there's a "them," a "not us," who are defined by everything "we" are not, and "they" are cast first as stereotypes and then as caricatures. Seeing outsiders as complete human beings becomes impossible, and empathy goes out the window right along with it. It happens so quickly that people don't see it coming until it gets way out of hand. We see this in nearly every social institution and it's toxic. Humans as a species need to figure out how to create belonging and community without feeling the need to set others apart in order to feel bonded to those close at hand.