help me understand why rascism occurs

as i've grown older i've tended to avoid asking many why questions cos the answers are too complex. but i'm interested in this one too much.

i think the reasons are probably different in different historical contexts. in the UK i can see (thought not totally agree with), why people are anti-Islam, with the attacks n media hype n so forth.

and i can see why older people looked down on blacks, cos when they entered the UK they came into the working class, and so the tendency for crime n so forth would have been greater, hence a bad reputation.

but now with loads of respected blacks in prominent positions, people are still rascist. is it just because they look different, and people dont generally lyk things that are different? is it as simple as that?
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Look you dumb worthless nigger, Niggers were never in the garden of edan. Niggers are Sin incarnate. They are the bane of mankind. The only reason you try and convince yourslef that whites came from blacks is because you know you are worthless slaves and thats all you will ever be. Do yourself a favor nigger and just accept it. Pick my cotton and you'll find yourself more happy than you are now. Everyone is happier doing what they were meant to do and by God you were meant to pick cotton.

Thank you.