The key to masculinity and vitality is the human growth hormone that separates the boys from the men. It plays a vital role in the creation of muscle mass and strong bones. It contributes to deep voice and even sex drive. Low testosterone in men is known as hypogonadism. As a product of a modern world made for convenience the male body does not go through nearly as much rigorous activity. The lack of daily exercise contributes to the decrease in male testosterone. It can hit as early as the mid 20s and steadily decrease each subsequent year.
The second theory is similar and is known as "evolutionary neuroandrogenic (ENA) theory of male aggression".   Testosterone and other androgens have evolved to masculinize a brain in order to be competitive even to the point of risking harm to the person and others. By doing so, individuals with masculinized brains as a result of pre-natal and adult life testosterone and androgens enhance their resource acquiring abilities in order to survive, attract and copulate with mates as much as possible.  The masculinization of the brain is not just mediated by testosterone levels at the adult stage, but also testosterone exposure in the womb as a fetus. Higher pre-natal testosterone indicated by a low digit ratio as well as adult testosterone levels increased risk of fouls or aggression among male players in a soccer game.  Studies have also found higher pre-natal testosterone or lower digit ratio to be correlated with higher aggression in males.     
Testosterone is a hormone that is produced in large amounts by males (and a little bit in females), in the testes and adrenal glands. High testosterone levels are associated with sexual performance, reproductive function, muscle mass, hair growth, aggressive, competitive behaviors, and other such manly things. Testosterone levels tend to peak at the age of 40, and slowly decline from there. Luckily, there are many things you can do to increase testosterone, so if you feel like your T levels could use a boost, you've come to the right place.