racial tolerance

It has long been known that positive interaction with members of other groups (ethnic, religious and so on) promotes more positive attitudes to others. Why, then, is it sometimes claimed that diversity poses a challenge to modern society, makes people withdraw from interaction, makes us less trusting of one another?
It may seem like common sense that people can be made less racist by simply moving to live in more ethnically-mixed areas
Do you think Britain is an intolerant society? With all the talk of "immigrants taking our jobs", Islam being "incompatible