It can be a number of things. A lot of guys grow up in an environment where they're taught that showing tender emotion is a "sissy" thing, a sign of weakness, whether it's by their parents, siblings or friends. Others have a hard time with it because it involves trusting the other person, and perhaps they've been burned by people in the past. Or maybe they feel like becoming vulnerable means a loss of control of any situation, and so they shut that off as a defense. Another possibility is that, since a large number of men have the ability to compartmentalize different aspects of their lives, some guys are maybe a little too good at it, and are able to shut the emotional parts of their lives off from everyone and everything else.
Because guys are people too.
but yeah, some guys do way better at it than others
some guys think it makes them weak , maybe? or look weak, at least?