Lower cl Egyptians aren't allowed to be with upper cl/ Normal cl?

Lower cl Egyptians? Well I recently found out that it's improper in Egyptian culture to like someone who is lower cl, why? I mean this guy my friend likes works in a shop down the road and he seems to flirt with her, but it's not proper? I don't know. When her father jokes about him she get defensive & all but she could never properly be with him, why? If you love someone is it wrong?