Why do women try to pull their friends away from men who approach them?

I've seen this happen so often. Let's say there is a small group of women, or even just two women, on a night out. And a man approaches one of the women and they both seem to be enjoying the conversation. Then all of a sudden, one of the women's friends starts pulling her away, or tells the man to back off. Even though the woman seemed to be happy talking to the man, one of her friends doesn't like the idea of them talking and tries to separate them. Why do women do this?

Why do women try to pull their friends away from men who approach them?
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