How did greed (aka self-interest) go from being one of the deadliest of sins to become the underpinning and core value of an entire civilization? That's the question that A. O. Hirschman attempts to answer in The Passion and the Interests, which traces the evolution of the intellectual concepts that undergird capitalism. Hirschman argues that greed came to be considered a counterveiling passion to the more destructive passions of lust and hatred and honor and so forth because of its predictability and presumed universality. Also, it's very insatiability came to be seen as a point in its favor, because the presumed fact that the desire for more wealth can never be sated means that avarice is always operating in all men, and can be used as a lever to rule over all men alike. As Simmel explained, "as a thing absolutely devoid of quality, [money] cannot harbor either surprise or disappointment, as does any object." Coveting money so easily beccomes a single-minded mania because it is never subject to the law of diminishing marginal utility: one ham sandwich is nice, but five of them is overkill. But a million dollars is nice, and five million is even nicer, and fifty million is nicer still -- as long as one doesn't try to convert money into specific utility. The lust for money is insatiable only when it is fetishized as an end unto itself and not a means to another end.
Reduced to his greed, man's psychology is rather simple, and manipulating him into participating in a social order becomes that much more plausible. To this end, one might conclude, man is henceforth encouraged in his greed at the expense of other pursuits. He is encouraged to fetishize money as an end in itself. Self-interest is held to be the only interest; all other interests ultimately can be reduced to it (a la Rochefoucauld's Maxims). From this point, self-interest can be defined as the only form of rational decision-making, and refusal to be greedy can be seen as a kind of insanity. Because the greed of a host of independent agents is seen to check and balance each other in the aggregate, and produce a growing and expanding society, refusal to be greedy is positively detrimental, subversive. But following one's greed is perfectly natural and excusable.