In short, is manliness something men have defined for themselves?
Here's my hypothesis:
Men are competitive and homophobic.
1) Things like eating and drinking becomes a competition and the guy who can eat the most meat or drink the most drinks must be the most manly
2) Gladiatorial sports, or sports that involve a lot of violence are considered manly because it satisfies some sort of competitive urge tha we have. We have to see some sort of conflict.
3) The stereotypical non-sensitive not holding back tears comes from homophobia. Being sensitive and emotional = being feminine and if you're a man and you have feminine qualities, you must be gay.