Why can't Roman Catholics take the bread and wine at a protestant church?

I do not understand this. I am a Roman Catholic, and my Grandpa was born in Sicily. Anyways, I was at CCD(sunday school) a couple months ago, and the Deacon, was talking about the bread, wine, and how much Catholics and Protestants don't get along... Then he said it was a sin to take the bread and wine at a protestant such as Methodist or Baptist, because we don't have the same believe the same thing on the topic. I thought he was crazy, I'm only 14, and couldn't understand what he is talking about. I personally don't think it is a sin, if I went to my friends church lets say Lutheran, and I took the bread, I couldn't see going to a preist and asking for his forgiveness. I think it is really selfish. Also, why do Protestants and Catholics hate eachother so much? In school we always talked about how Catholics weren't aloud to come to the New America. Some people look at me funny when I tell them I'm catholic, because I live in redneck ville GA, and everyone here is either Methodist or