Guys, What would make you protective over a woman you aren't in a relationship with?

So like the question said what would make you protective over a woman you aren't in a relationship with? Im not talking about female relatives either. I've heard many men say "you can't be friends with women" so if you're protective of someone there must be another reason.

Also I'm talking about actual protective like "hey, I'm keeping her from getting stabbed" or that guy is being a jerk to her I'm stepping in.

Guys, What would make you protective over a woman you aren't in a relationship with?
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