Most Helpful Guy
a lot of people will say "society" or "afraid of being hurt" and, while these have a degree of truth, the underlying reason is instincts. Males are both predators and hypersensitive to being prey. When we emote, we give potential enemies information about our weaknesses. This gives power to those who can threaten us and thus assists them in defeating us and/or weakens our ability to protect that which is important to us. In particular, as we get older, we realize that women "share" too much for our liking. If a man is fully open up to his woman and she broadcasts to her girlfriend who then broadcasts to her man or other girlfriends (and their men), then our weaknesses become well-known. That puts us on the defensive and elevates our stress. So, we keep our mouth shut for the most part unless we have complete proven trust in someone else. This is why infidelity is such a big issue with men; it isn't that your getting fucked by another guy - it's that your are intimate and likely sharing with that other man. In other words, you may be revealing our weaknesses to him which he can use against us (and you).
So, in short, men don't trust easy and each time our trust is violated, we clam up that much more. Boys can be chatterboxes, but men aren't (unless we're fudgepackers) because we know better.0