Read Robert Putnam. People love a sense of belonging and it's embedded in our genes.
http://time.com/5095903/genetic-similarities-friends-study/
http://www.bu.edu/sph/2017/04/07/married-couples-with-common...
>It's been my observation that the people who push back against the idea of diversity the most tend to be quite miserable even when living in homogeneous communities.
There can definitely be miserable people who are looking for a scapegoat. It also takes a certain level of desperation to be vocal about it today because it's so taboo.
On the other hand there are people like me: anonymous, have traveled the world, lived in rural areas and cities, have a masters from a top 10, have a good job, etc. who look at the data and go: "Oh, the data is indicating that this recent configuration of humans isn't as good as it could be." You only have to go back 2-3 generations for homogeneity to be the overwhelming norm--were they all miserable?
Portraying people who are against diversity as ignorant or miserable doesn't disprove the fact that if you take a homogenous, high-performing population and flood it with diverse, lower-IQ, culturally-incomptable, more violent, less-productive people the nation will become worse.
Simple question: would Europe be better or worse today if there were only Europeans in it?