Humans are irrationally xenophobic because we are irrationally loyal.
This thought occurred to me, while a friend related the story of a organization he had worked for that seemed unable or unwilling to fire the very person who brought it to the edge of extinction. They could not do it, it seemed, simply because he was a nice guy and well-liked by all. Because of their fondness for him, they were unable to terminate the relationship.
I think now that humans are reluctant to welcome newcomers into their social units — especially those who are not familiar or similar to them — because once admitted they will be very difficult to expel for reasons of attachment and dependence. New people — especially the unfamiliar and dissimilar — may upset the status quo, for better or for worse, and, for reasons of attachment and dependence, the group will be hard pressed to expel the disruption once s/he is accepted as “one of us”, especially when the disruptor acts like s/he is one of us. Rather than assume a risk they won’t be able to expel, people and groups seem to prefer to refuse entry or to make it as difficult as possible.
Do you really want to associate with people who are willing to fawn or bludgeon their way into your company (pun intended)?