Why is being vulnerable with a girl advised against?

I have seen this a lot on reddit and also sometimes on here, especially by pickup artists and redpilled people.

they say to not be vulnerable with girls because it shows you as weak to a girl.

aren’t girls just humans like us? Why is showing emotional vulnerability seen as weak and bad? I would love to see a girl be vulnerable with me.

Why is being vulnerable with a girl advised against?
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