What makes discrimination acceptable?

The thing about discrimination is that everyone can find a reason that they are discriminated against. I could go into a jewelry store and say they are discriminating against me because I'm poor and they make the jewelry too expensive for me to buy. However, it's never a good idea to ume that people are discriminating against you. Especially if it's something like red hair. It's more sensitive when you look at the history of slavery and whatnot.