From “Seven Deadly Sins: The Biology of Being Human” by Guy Leschziner
One very easy answer to this question is that being male is the strongest factor, that testosterone is the explanation for aggression. In almost all mammals, males are more aggressive than females (notable exceptions being lemurs and spotted hyenas). Testosterone enhances sensitivity to social threats. It activates centres of the brain involved in aggression and the processing of threat.
It may be that surges in testosterone associated with competition or threat may facilitate aggression to deal with those threats. Indeed, testosterone levels have been shown to be directly affected by competition in males. Levels rise prior to competition, in anticipation of conflict and swing wildly according to the outcome. If you win at wrestling, tennis or even chess (an activity not renowned for its sexual allure), your testosterone levels rise; if you lose, they fall. These findings have led some researchers to argue that the evolutionary function of testosterone is related to competition between males, with females being attracted to winners of these competitions. And if indeed this is the case, that mating success is the key outcome of the relationship between testosterone and aggression, then testosterone levels should fall when you have mated. This relationship between testosterone and mating has borne out: testosterone levels fall when a man marries, but rise again after a divorce, when he must compete again for female attention.