it's kinda strange because the US has a culture of unity above race, religion, etc. but in practice there are a lot of tensions between groups. here in Spain nobody cares.

This probably depends on where you live. In my area, no one really gives a shit. Where I grew up... It was harder when I dated a black girl in my teen years. I didn't understand it then and I still don't. And weirdly this issue isn't limited to one race. The issue differs by area in my experience. Maybe our melting pot nature just leads to conflict like this? I would guess we are one of the most diverse countries. Mix in some historical issues and the fiat world leader and problems abound.