I think it goes back to the human habit of creating in groups and out groups. It comes naturally to chimpanzees and it seems to come naturally with us. Certain people in either country use this unfortunate tendency to their own political advantage to gain power and status. The most barbaric example of this sort of behaviour being the treatment of jewish people in Europe by the Nazis.

Fortunately there are a lot of people in this world building bridges between different cultures. Peacemongering if you will. I hope it will be enough.