Though Ralph is initially elected the leader of the boys marooned on the island, the more aggressive Jack is able to eventually take over. Jack accomplishes this by exploiting the fear of the other boys. They're in an unfamiliar environment. Dangers are all around them to be sure, but Jack plays upon childish suspicions and anxieties, such as fears about "the Beast" supposedly lurking on the island. He creates his own island subculture centered around hunting pigs, giving his side the appearance of competence and physical prowess compared to Ralph, who tries to have a more levelheaded perception of island life. However, Ralph's methods do not have the immediate satisfaction that Jack provides.
Jack is also able to wrest power away from Ralph because he gives the other boys something they most desperately crave in their situation: the illusion of control. Aggressiveness is usually a manner of trying to gain control over someone or something: think about the number of people who hit or shout at an appliance when it does not work. By channeling an innate sense of aggression, Jack allows the other boys to believe they too can conquer the wilderness of the island through hunting and other such rituals.