In addition to the factors listed above, there are still other reasons for 'clamming up' at emotional responses. It has much to do with the conditioning discussed by Sharon. Only, instead of environmental, this deals with interpersonal interactions. For example: woman is upset, man tried to show his emotional side, woman misinterprets and goes ballistic, man decides he should've kept his mouth shut. And the cycle repeats.
There is an old saying: "A close mouth gathers no foot." Unfortunately, men are unrivaled at this exercise that usually only the most flexible are able to attain. But it's not all on the guys. There are many women with excellent memories for certain details, and will keep those filters of past male mistakes in place, screening anything said to see if it's an old blunder, or a new one.
Allow me to flip this, as I personally know and have heard of many men who do this very same thing to women. The men are emotional, the women burned, and the guilt of not being emotionally open is heaped upon the female's shoulders.
All of these cycles will continue to repeat, recycling the hurt and distrust until forgiveness is freely given and received, for once forgiven, and I mean truly forgiven, the past wrong, hurt, misinterpreted word/action/thought are all no longer remembered, in the context of being recalled for future conversational ammo. Then, and only then, can truly open conversation, including emotional, be expressed without fear of retaliation/retribution/misunderstanding.
I realize this is idyllic, and not likely to come to pass in any of our lifetimes, but all changes, all movements begin with one life, which affects others.