And Sweden is NOT diverse as US, Sweden has as the "biggest minority" the Finnish (that are not that different from Swedes), any other population is very small except on some specific cities.
US has a great population of people from several backgrounds.
Also the same is in my country (Brazil).
Here, I see people being much more lax with their security when everyone near them are of the same ethnic background, be it white, black, native american, whatever.
It is just that everyone is born to not trust strangers, foreigners, etc... And "race" is a very quick way to assess that.
Not that I think we should go hating each other or anything like that, but it is how it works, and it is very visible, the more mixed a place is, the less people trust each other, with some specific exceptions (ie: places that are truly cosmopolitan like Universities or some workplace cities).