I think part of can be explained by the amount of isolation a given society experiences. Once one society comes in contact with another, it discovers things that other society innovated, and offers what it has innovated in trade. So innovation and product become prized. The strong within the society organize their society to produce and innovate more, so more goods can be obtained from their trading partners, and they organize defenses so they cannot be stolen from them or so they can not be enslaved to produce for others. That in turn leads to innovation in warfare, and the discovery that warfare can be substituted for technological advancement and trade goods bartering. As the more advanced societies and more warlike societies come into contact with weaker societies, they absorb them or colonize them.