Societies amalgamate into more complex, centralized societies usually in response to outside forces or factors. As Diamond argues, "amalgamations never occur by a process of unthreatened little societies freely deciding to merge." Rather, such societies come together due to a threat from an outside force or as a result of invasion or conquest. The examples he cites include the Cherokee, who emerged as a nation in response to constant pressure from colonists in the eighteenth century and strengthened their bond in reaction to American attempts to strip them of their lands in the nineteenth century. Sometimes, these societies are created by conquest, as a stronger power unifies them into an empire or a political whole by taking over each of them through military force.

So these are the (fairly obvious) reasons smaller human societies merge into larger, more complex ones. Diamond's point, though, is a deeper one. He argues that in order for these external factors to lead to amalgamation, other factors have to be in place. For example, societies have to have sufficiently dense populations to enable such mergers. If not, then peoples are very difficult to merge through conquest, because they can just move away. So, as Diamond writes, these amalgamations are, more fundamentally, the result of deeper causes:

Thus, food production, and competition and diffusion between societies, led as ultimate causes, via chains of causation... to the proximate agents of conquest: germs, writing, technology, and centralized political organization.

In other words, food production and competition and diffusion between societies had to exist for amalgamation of simpler societies into larger, more complex ones to take place.

Guns, Germs, and Steel by anthropologist Jared Diamond is explicitly intended to answer a question posed to Diamond by Yali, a local leader in Papua New Guinea. That question was,

Why is it that you white people developed so much cargo and brought it to New Guinea, but we black people had little cargo of our own?

Diamond notes that people he has met all around the world both from advanced and less developed cultures are equally smart and hardworking, and thus, the impetus for some societies evolving into large, complex ones and others not doing so does not depend on the innate capacities or habits of the people comprising those societies, but instead on environmental and geographical factors.

Diamond argues that several environmental factors facilitate the growth of complex societies. The main one is the climate and availability of animals and plants suitable for domestication. Agriculture allows for the "neolithic transition" into a society that produces enough food to support population growth, urbanization, and specialization of labor. Societies with these advantages create virtuous cycles in which increased efficiency in food production increases population and technological innovation, which in turn allows for more efficient food production. This grants military superiority, allowing such societies to conquer or assimilate their neighbors.

Also, Diamond argues that an East-West communication access and trade routes allowed free interchanges of ideas and goods, which accelerated the process of development in Eurasia.

In the classic work Guns, Germs, and Steel: The Fates of Human Societies by Jared Diamond, the author argues that environmental circumstances rather than differences in intellect or genetics are responsible for the survival and dominance of some societies rather than others. Simple societies can evolve into more complex ones only where conditions to produce large quantities of food to nourish dense populations are present. If the flora and fauna of an area offer this possibility, then Diamond explains that "intensified food production and societal complexity stimulate each other, by autocatalysis." This refers to a chemical reaction in which the product of the reaction works as a catalyst for further reaction.

In other words, food production leads to sedentary living and higher population, which necessitates societal complexity, which in turn leads to greater food production, and so on. Societal complexity and centralization enable public works such as irrigation systems, long-distance trade, and economic specialization. Advanced food production leads to stored food surpluses. This in turn allows social stratification, or the forming of complex societies with chiefs, bureaucrats, scribes, craftsmen, and farmers.

However, Diamond explains that the amalgamation of smaller societies into larger ones does not come about peacefully and freely. Smaller societies are generally jealous and suspicious of each other. As Diamond writes in chapter 14 of the book,

Amalgamation occurs instead in either of two ways: by merger under the threat of external force, or by actual conquest.

To illustrate the amalgamation of smaller societies under the threat of external force, Diamond uses the example of the thirty or forty disparate Cherokee chiefdoms in the eighteenth century. Threatened by whites, they amalgamated into a single confederacy overseen by a decision-making council. In similar fashion, the American colonies united into a more complex society when faced with the external threat of Great Britain.

Amalgamation by conquest involves one chiefdom taking the initiative to conquer other chiefdoms and unite them by force. The example Diamond uses is the rise of the Zulu state as one chief conquered others in southeastern Africa in the eighteenth century.

The answer to this question can be found at the end of Chapter 14.  It starts on p. 289 in the paperback edition of the book.  Diamond says that this amalgamation never happens voluntarily.  Instead, it happens through coercion. 

The first way in which small societies amalgamate into bigger ones is through the “threat of external force.”  In other words, a small society recognizes that it is in danger of being swallowed up by a bigger society.  In order to prevent this from happening, the smaller society joins together with other small societies in hopes of being able to fend off the larger society.  Diamond says that this happened with Native American tribes that amalgamated to try to resist white incursions.

The other way in which small societies amalgamate is through actual conquest.  Diamond describes this on p. 290.  He says that a small society can become strong and can go out and conquer other small societies.  He gives the Zulu state in southern Africa as an example of this process.

Thus, small societies only amalgamate into larger ones when they are forced to do so.