Why do most fathers say "Boys don't cry"?

I am a pregnant wife with a caring loving husband. He never played sports and was a nerdy boy when he was younger. But my he always says he is going to make sure our son has a great life growing up, and that while he is parenting the boy, he will be nothing like his father. I asked him what he meant and he said that "My father always told me that if boys cry there babies or losers, I had to hide so many emotions, I don't want it to be that way with our son." (and yes we know it's a boy) But I have heard stories like this before with tough fathers and wimpy sons so why do most fathers say boys don't cry? Any answers appreciated :D