It's societal and neurological. Male brains are geared more towards actions and in depth processes than they are towards emotional and sensing quantities. This being said, men can still possess the ability to process emotions like a woman, however it's far less common in normal men. Gender identity plays a huge role in this as Men who identify male, they tend to through their identity emulate other men. This is where the social aspect of it comes into play. Men base their ideas of maleness on their fathers and male family members then later on the personifications of other men, editing here and there until they can or seek to find their medium. Men become masculine and out of touch with their emotions because (in part) the emulate the behaviors of other men.
Now, the interesting thing about this is the more deeply rooted and harder to control aspect of this... Testosterone. Testosterone is know to have a deadening effect on emotional response, and depending the it's lows and it's highs, the emotionality of men fluctuates, in most cases almost daily. Men tend to peak in the morning, and throughout the afternoon and fall off in the evening unless sexually stimulated. However, people on testosterone can also it too high of an amount can result in loses of control of anger, while all other emotions are silent. But fundamentally, Testosterone in men produces a reaction that heightens logical quick response skills and turns emotions into off mode. It's more like it just dulls, then turning off. So, the combination of conditioning, gender identity, hormones and normal male neurological structuring results in a being with slightly more aggressive quick react processing, and less sensing and less emotional connectivity.
This, however, doesn't mean that men can't become in tune with their emotions, but just not on the same level as women. If society expected men on a whole to be more sensitive, the later societal influence wouldn't worsen the overall built in aspects. Men have plenty of emotions, but are configured to act rather than feel. In the Myers-Brigs type indicators, Men tend to be SP and ST and women tend to be NF or NT. Women are sensors (or iNtuitive) more often, and can pick up on others emotions, but some women break this pattern. However, the general consensus puts women as intuitive, and feeling people. There is an extensive middle ground of mixes; as far as NF/NT versus SP/ST it is predominantly S types for males, and N types for women.
MBTI, Psychology studies, and personal experience, plus my own inborn woman's intuition, not to mention my observations of people.