Why do females give each other dirty looks?

It is because they are in competition with you (and also because perhaps you are too sensitive)(and perhaps because you underestimate yourself). Think of how much time many women spend every day trying to make themselves "attractive" to other people.

jealousy .You can be their next competitor even if you are average female

Women are very competitive. Simply observe how they look at your shoes, your handbag, how your make up is. If your looking good that especially peeves them.

I learned many years ago, I can have one girl work for me. Have two girls, and you have competition. Three girls or more is a DISASTER ! Two or more gang up on one, and make her life Hellll. So for the last 10 years I have only had ONE girl working for me. That because I fired the other two who ganged up on her.

This is typical woman's behavior. Guys do not act like that.

I have never observed women doing that. When someone gives a person a dirty look, it's usually because the person is doing something annoying- talking loudly, smacking gum- or because they're dressed like a prostitute. Are you sure you're not just being paranoid?

Sheer envy.

Because for the most part women are jokes...