From ages 12 to 14 (in the late sixties) I lived in Tucson Arizona. Everything was fine until the other kids realized I was Jewish. They were actually looking for my horns. Some had never seen a Jewish person. Having grown up in NYC, which is such a melting pot, and having had friends of all ethnic backgrounds, it was shocking to me to see such ignorance...especially since I had come from such a progressive and socially active family (including a mother who had me marching with Martin Luther King before I could even walk well on my own...). Stuffed inside our daily papers was hate literature of the John Birch Society, the American Nazi Party and the Ku Klux Klan... some contrast to what I was used to. So it wasn't surprising that when the parents of the white kids found out that I was a Heb they weren't allowed to socialize with me. All of the friends who stuck by me were Black, Mexican, and Indian.
The day Dr. King was murdered there was mayhem in my school. A lot of white kids got slashed. A group of black girls (who didn't know me) surrounded me outside of the school and began taking razor blades out of their afros. Just then, my friend LaJuana Lincoln, aka "Tia" ... (and I'll never forget her name because she saved my life)... burst into the circle and screamed "Get away from that girl... she's not white, she's Jewish!!!"
She considered me a sister.
That experience was very powerful for me. I got to experience prejudice (and solidarity) first hand.
I think what causes prejudice is insecurity, jealousy, and, of course, ignorance.
As I said before - and I must repeat - we have a lot of growing up to do.