It might surprise you to know that boys are more emotionally expressive than girls before age two. That difference does not last very long, as many boys are made to feel ashamed of themselves for violating masculine norms by expressing their feelings. And this prohibition against being sad, fearful, disappointed, lonely, or any emotion other than anger follows many into a stoic manhood.
Why is this a problem? As we see it, there are several serious consequences to this limited emotional expression among many men. The most important of these is the propensity for violence, which men engage in nine times more often than women. A direct result of the ability to shut down vulnerable emotions is the inability to empathize with others. And violence is, first and foremost, a failure of empathy. If you have a compassionate understanding of how your actions can hurt another person, you would be very unlikely to cause intentional harm. Men who embrace this aspect of cultural masculinity cannot empathize with another person's emotional experience because they have no point of reference in their own minds with which to understand it.
