A happy society is one where people share an ideal of what should be, and thus they act toward similar goals without a police state enforcing rules on them
Diversity ruins this idea.
Diversity puts people in an ugly position: either disregard your native culture and become an anonymous person who gets his/her culture from shopping malls and TV, or keep your native culture and be an outsider.
Conventionally, criticism of diversity is labeled "racism," which is a logical fallacy that excludes the possibility that someone could criticize diversity for some reason other than racism.
The realistic view is that diversity of any form does not work. Racial, religious, ethnic, cultural, class and even values. Mix the different and you make them all opposites. They're different for a reason: over history, the human race has branched out into many unique tributaries.
However, it benefits our overlords to have a society that is not united. We fight among each other and ignore the long-term problems that our society is piling up by ignoring the obvious.
Diversity is thus, like many other gestures of dying societies, a surrogate and a substitute but not action that can actually save us from our own decay.
Like many others, I've watched Rome wind down and begin to burn and I realize that most people have a singular response to this, which is strong and violent denial. This is why they call you racist, crazy, etc. if you criticize diversity, the welfare state, democracy, consumerism, egalitarianism, police actions or any of the other fictions our society has come to depend on like crutches.