Inborn tendency: Aggressiveness is an inborn tendency among human beings (as it is in animals). Biologically, this inborn tendency may be meant for selfdefence.
Physiological mechanisms: Aggression could also be indirectly triggered by physiological mechanisms, especially by the activation of certain parts of the brain that play a role in emotional experience.
Child-rearing: The way an individual is brought up often influences her/his aggressiveness. For example, children whose parents use physical punishment end up becoming more aggressive than children whose parents use other disciplinary techniques. This could be because the parent has set an example of aggressive behaviour, which the child imitates. It could also be because physical punishment makes the child angry and resentful; as the child grows up, s/he expresses this anger through aggressive behaviour.