Because we internalize more than you. It's not healthy to do such and will mostly be let out in fits of anger after a boiling point.
I agree with this as well. Even for guys with enlightened parents who taught him "it's OK to cry," there's still a deep and pervasive message in our culture that guys are supposed to be cool, calm, collected, and in control (not only of themselves, but of the situation). Men are also supposed to be tough, to "be a man" and take what life throws at you, to play the hand you're dealt. Of course, the reality is that there are many things we can't control, especially other people. Consciously or not, I think that frustrates a lot of guys, makes them feel vulnerable and insecure, causes them a lot of stress, and generally drives them nuts. Frankly, I think a lot of guys feel like the world is pissing on them (or worse), which is not how they expected things to be, and they don't know how to deal with it. Most of us are so cut off from our own hearts, we don't even know what it is that we're feeling, or why we're feeling it, so how can we possibly talk about it? Then the women in our lives try to find out what we're feeling, and that just makes us feel nagged, pressured, and more stressed out.
I also think our culture displays anger a positive emotion that men should own and embrace. In most of our stories, there hero (usually male) is either on a quest or he's fighting for something (truth, justice, his family, his home), and he finds his strength in a kind of righteous anger. It's when he stops being afraid or unsure of himself, and when he becomes a tough guy and says "come on, let's do this" or "let's finish it" or "go on, make my day" that he finds an almost supernatural ability to overcome any obstacle, vanquish all evil, and save the day.
Unfortunately, that doesn't happen in real life, either. It's a lot uglier and messier than that.
When we reach our limit, when it all just gets to be too much, we sometimes embrace our negative emotions instead of bottling them up, the rush of adrenaline (and whatever other hormone are involved) makes us feel strong, powerful, and aggressive--sometimes to the point of being totally irrational and unable to contemplate the wisdom of our actions and the possible consequences. We fell like we can do anything, take on anything, and it feels good, in a sick, terrible way. (Frankly, it's not all that different from being sexually aroused.) It feels good to release all the hurt, anger, frustration, and to vent it towards someone, to take it out on someone, and to make someone else hurt--especially if we perceive the person in question as a source or cause of our pain.
Sometimes these are violent outbursts, sometimes they are hurtful words, and sometimes they are channelled in relatively safe, healthy ways (e.g., working out, going for a run, hitting a punching bag, going to loud concert and dancing in a mosh pit--whatever works). One of the worst things you can do to a guy is to deny him an effective outlet, but sometimes our partners do that.
Of course, it would be better if we didn't let things get to that point, if we had a more caring, nuturing, mutually supportive culture and if there were plenty of positive, constructive ways to let out our frustrations.
My generation isn't as bad as my father's and his wasn't as bad as his father's. But we're still a long ways away from having a successful new model of masculinity. In a way, we're currently in an awkward transitional phase which might be a sign of progress, but which has also left us confused about who we are, what we're supposed to be, and how we're supposed to relate to others.
Unfortunately, as a culture, I also think we're a lot more selfish and self-centered than we used to be, and if we don't get it under control our problems as individuals and as a society are only going to get worse, no matter what progress we make in other areas.
