Of the primary emotions fear is the one that bears most directly on survival. Children show fear. Adults try not to maybe because it's shameful or in some circumstances dangerous. The fear response is automatic though and your body runs through its reflexes whether you want it to or not.
I think masculinity is bravado against the mystery of the universe of women. It's just a fear of not knowing what women have that's so powerful. It's this shield they put up to try to get closer.