A man´s attractiveness is linked to his confidence/dominance/indifference.
Men are supposed to be big, dominant, muscular, able to kill monsters, defend a family, defeat other men. And happiness as a man is linked to doing these things. But how much time do you have to smile?
Sort a bunch of male pictures and rank them. More dominant, stronger first and weaker last. Check where the smiles rank in that order.
Then, smiling is a way to seek approval / dissipate tensions / signaling peace. Smiling says: “Im not looking for conflict”
Men are supposed to be busy on a big scale mission saving the world and defusing a bomb or fighting the enemy or hunting moby dick or whatever. If you get to smile on top of that, it better be a cocky smile.