Dating in different social classes

I have one question for all you preps, middle class ,and the lower class people. why does your boyfriend or girlfriend have to be in the same social class as you? For example I have study hall with two individuals that i thought would make a cute couple. The boy liked the girl an the girl thought the boy was cute but they wouldn't date because they were in different social classes. Since they were in different social classes they knew nothing about each other but they live two blocks away from each other. Please tell me why this is.