Why do we like the "musky" smell of our male partner?

I've asked a few girls if they like or dislike the smell of their man after he works for a few hours or at the end of the day when he gets home. All girls have said they like their partners smell. But if another guy is sweaty and musky (non-partner) then it's kinda gross or unattractive.
If it was a "animal instinct" kinda thing we would like that smell from every guy, not just our own man.
So explanations?
Also is it weird that I like putting my face in my bf armpit?
Why do we like the "musky" smell of our male partner?
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