Mainly, its because we're not allowed to have them, or at least display them.
We're not allowed to be upset, or anxious, or happy, or scared, or any number of other emotions. Compared to a lot of men, I'm quite in tune with my emotions, but that's in spite of my upbringing and the world around me, not because of it. I was berated by my Dad for crying, for being worried, for not being confident. I was dumped by my girlfriend for admitting it was slightly scary walking down a dark alley in an unknown city at night. I was clipped round the ear and called a big girl by my Gran for daring to say a view was "pretty"
We're allowed anger, we're allowed irritation. These are expected of us, so thats what we turn negative emotions into. I was so cross after my Mum died. At the world, at everyone, over nothing. And by god I tried to hide it, because DP, DD, family, friends, none of them deserved my ire. But absolutely everyone and everything just pissed me off so much! I wasn't really cross, I was sad, I was grieving. But these are bad emotions, emotions we're taught not to show, so they get bottled up and come out in other, more unhealthy ways instead.
I think things are changing, boys are encouraged to express their emotions more these days. I hope it works, and that we're raising a generation of men who are more open with their emotions, who are able to release them without them coming out as anger.