History of Feminism

Men and women tell boys to be "men" when they don't have the time, energy, or know how to help or take care of the boy themselves.
Since the welfare of the boy is the responsibility of the adult, the adult, in seeking relief from responsibility, wants the child to grow up.
As a result, the phrase is probably most often uttered when the adult is under pressure, frustrated, or feels overwhelmed themselves, so a sharp or disdainful tone might be used when commanding boys to be men.
The command implies that the boy is not measuring up to expectations and should not need the attention or help of an adult.
Being told to "be a man" is far worse than being told to grow up (which all children must eventually do), because the added anti-female hodge-podge of messages implied by telling boys to be men is confusing for developing personalities.
I think that "real men" don't talk about how being told to be men affects them because "real men" don't share things like that. It is their own cross to bear.