Why is it so wrong for guys to cry?

I know that us guys have been "raised" to think that crying is for the weak and defenseless, and that us men shouldn't cry. Even when I'm on the verge of tears, at school, or even at home alone in my room, I tell my self that I shouldn't cry. I'm a guy, and guys don't cry. I just let it out in the shower, where no one can hear me, and sometimes, I keep things bottled up so much, that I almost end up shouting in sadness that I have to cover my mouth with a towel. But it still confuses me. Why are men so forbidden to cry. And how would people nowadays react to such a thing, considering how lenient some of us are now to people and different races.