Maybe he has a tiny penis and he's embarrassed by it?
A true man (or woman for that matter), a confident person, could not care less about how others define themselves. The fact there is so much of a need to show everyone what a true man supposedly looks like and does suggests a lack of confidence in oneself, a need for everyone to be like him, so he can belong. To a confident person, it's ridiculous, but to someone with that need, it's a defense mechanism.
If my (theoretical) son decides to play with a Ken doll instead of a John Cena figure, who really cares...you know, besides those who want to force an arbitrary definition of manhood on everyone? It has no effect on anything and my child is four. Who cares?
But to those with a tiny penis, they feel like less of a man. It's silly, but it's well known. So in order to convince everyone they aren't less of a man, they shout about their arbitrary definition of manhood, seeking to redefine masculinity into something they can control, as opposed to penis size which they cannot control.
All of this brings me back to my original point, which is that it is my belief those who shout the loudest about how they define what it is to be a man tend to have the tiniest penises. If one already feels confident in their manhood, there's really no need to try and convince others to think about masculinity the way they do.