I'm guessing it's due to a beard essentially being the ultimate sign of masculinity and them wanting to convey their manliness with it. I don't blame them, but not all men are created equal when it comes to growing a beard. I for one couldn't grow a beard if I tried. Can't imagine myself sporting one anyway.
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“They laugh at me because I'm different; I laugh at them because they're all the same.”
― Kurt Cobain