According to a paper by N. Christakis and J. H. Fowler, “Friendship and Natural Selection,” Proceedings of the National Academy of Sciences, vo. 111, 2014, pp. 10796-10801, there is a genetic basis to friendships, with ties not only being to similar people, refuting the ideology that opposites attract, but this similarity goes as deep as the genes:
https://www.pnas.org/content/111/Supplement_3/10796
“More than any other species, humans form social ties to individuals who are neither kin nor mates, and these ties tend to be with similar people. Here, we show that this similarity extends to genotypes. Across the whole genome, friends’ genotypes at the single nucleotide polymorphism level tend to be positively correlated (homophilic). In fact, the increase in similarity relative to strangers is at the level of fourth cousins. However, certain genotypes are also negatively correlated (heterophilic) in friends. And the degree of correlation in genotypes can be used to create a “friendship score” that predicts the existence of friendship ties in a hold-out sample. A focused gene-set analysis indicates that some of the overall correlation in genotypes can be explained by specific systems; for example, an olfactory gene set is homophilic and an immune system gene set is heterophilic, suggesting that these systems may play a role in the formation or maintenance of friendship ties. Friends may be a kind of “functional kin.” Finally, homophilic genotypes exhibit significantly higher measures of positive selection, suggesting that, on average, they may yield a synergistic fitness advantage that has been helping to drive recent human evolution.”
I put it like this: that in general, friends are part of our racial family or tribe. Humans can recognise individuals of their own race much better than individuals of other races: E. A. Phelps, “Faces and Races in the Brain,” Nature Neuroscience, vol. 4, 2001, pp. 775-776; E. A. Phelps and L. A. Thomas, “Race, Behaviour, and the Brain: The Role of Neuroimaging in Understanding Complex Social Behaviors,” Political Psychology, vol. 24, 2003, pp. 747-758.
All the more reason to oppose mass swamping by immigration, which transfers once familiar social landscapes, into alienated deserts.