Why do men hide there emotions?

A week ago me F23 and my best friend F22 went on a long road trip, with us came her younger brother M17, a thing about my best friends family is that her parents are divorced and his father isn’t really the best, so they live with there mother.

We decided to cut the road trip in half and get a hotel room for one night and continue the next day, my best friend slept early, while me and her brother sat at the balcony chatting for a bit, and as time past our conversation went deeper.

He was telling me how his parents divorced shattered him, and how bad his father is, my best friend always talks about it but this time it’s different. He told me he NEVER talked about it, and I am the only person in the world he felt safe telling this to, he was crying as he was telling me how he has to keep his emotions in check and seem strong for his family.

He tells me he cries almost every night because that feels better when he wakes up, when he was done he hugged me like a little kid hugging his mother with his eyes filled with tears he told me that he felt safe telling me this and not to mention it with his mother or sister.

A week later I am still thinking why is this system built in for men to not openly show there emotions like his sister does?