Because women, like all Humans, cry when they're in a lot of pain. Men cry if you put their nuts in a vice and tighten it but good. There's no shame in it.
Yes, it's a massive, incredible, unbelievable gift and I am infinitely envious of the capacity of women to bear children, but that doesn't make it less painful.
Me, I think nature laughs like a mad bastard with several thousand tons of dynamite every time a woman gives birth. I have NO idea how something so vital to our survival could be so damn painful.
('Course it could just be that women are weak, too, but they're allowed to bitch when you say stuff like that these days, so best to keep mum. )