Because we made all this effort in our full-time job to look attractive to you. And if we're rejected that makes us think our effort was wasted. It makes us feel ugly and unattractive. I think it depends on the reason why you rejected us. At least for me, if a guy rejected me because of my personality then I really would care less. But if he rejected me because of my looks, well then that really sucks cuz looks is something I can't change. It's the same pain you guys would get if a girl rejected you because of your small penis. We care about our appearance why more then you can imagine, since we know guys are visual. And therefore we're almost sure that we're rejected because of our looks. But I'm never mad at a guy who rejected me, I'm just sad.
They've been told since birth that they are princesses, special snowflakes, and that any man would be LUCKY to have them. That is reinforced by how society kisses their asses.
So when you reject one, it brings everything she's ever known into question.
People do not like having everything they know brought into question. They react... badly... to it. Very badly. At best, they'll verbally attack the person responsible, calling them names. At worst, they'll physically attack the person responsible.
For all the shit we talk, we as a species are incredibly stupid, narrow-minded, and primitive. But gods help you if you see that and understand it.