Boys hear it from the day they're born: "Man up," "Men don't cry," "Learn to be a man." All these phrases mean that men are not allowed to show emotion, that they have to do "masculine" things, like not playing with dolls and shooting guns instead.*
*Indirectly the reason for gun violence among men, in my opinion.
Society has instilled in men the idea that they can't cry or show emotion, that they're supposed to be tough. This toxic masculinity causes problems in their day to day lives.
When a person has to hold in their emotions, it builds up over time and eventually becomes too much, and they crack over a little thing. Imagine a man doing this his entire life. Imagine holding in your emotion, because you have to by the rules of society, or else you'll seem weak, because men are not weak. Imagine being told to "man up" your entire life; it gets frustrating. Little things set you off.
When humans bottle up too much emotion, they'll take it out on whoever's close to them at the time that they crack. Unfortunately, this person may be their partner. One little thing cracks in a man, and before you know it he carries out violence toward his girlfriend/wife.
If we as a society started teaching boys and men that it's okay to show emotion, that they can play with "feminine" toys, perhaps domestic violence, and violence carried out by men in general, could stop.
Next time you want to tell your son to "man up" and shame him for crying, think about how that may affect him in the future.