Under capitalism labor power and wage labor are treated more or less as "true" commodities: over time the price of labor power tends towards its cost; a person who exchanges her labor will receive not what she spends — the hours she works — but rather the lower cost of producing those hours.
Capital has all the features of a commodity: It has a cost, it has use-value, it is exchanged, yet the price of capital does not tend towards its cost. Rather, the capitalist expects a permanent and continuous rate of return, a price that always far exceeds the actual cost, even when adjusted for risk (and the risk is disproportionally borne not by firmly entrenched capitalists but by those trying to enter the capitalist class).
Human labor always produces a surplus — a tiny surplus under primitive conditions, an enormous surplus under more advanced conditions — but where does this surplus go? If everything is exchanged at its underlying cost (i.e. the cost of producing labor power, the cost of producing the labor that produces the capital) then nobody would get the surplus, which is obviously nonsense.
The short answer to "Who gets the surplus?" is: the man with the gun. The longer answer is that the allocation of surplus is not at all an economic question, it's a political question. The way that human beings actually construct their society by exchanging and promulgating ideas determines how the surplus is allocated.
I'm not saying its necessarily the best way to do things, but under relatively primitive physical conditions (i.e. under human- and animal-powered peasant agriculture), it makes a certain degree of sense to concentrate the surplus into industrialization, under the control of those who can provably create successful industrial enterprises. Social constructions that privilege the owners of capital thus have a powerful "statistical gravity" — at least relative to social constructions that do not tend to concentrate capital into industrialization.
Thus the rise of capitalism goes hand-in-hand with social, political and legal constructions that allocate the surplus generated by human labor to the owners of capital.
The most important of these social, legal and political constructions under capitalism are property rights. Property rights exist as "memes" (actual ideas in peoples minds): Actual human beings believe that Bill Gates deserves his tens of billions of dollars; every day he personally creates more wealth than ten million of the world's poorest people </sarcasm>. Property rights exist as legal constructions (statutes and precedents), and support for property rights are directly and indirectly "baked into" the Constitution and our governmental institutions.
But what specific property rights exist? Or, more pointedly, what specific property rights do not exist? Most notably, a person's own time is not her own property: a person does not own her own labor; she owns and can exchange only her labor power, her ability to produce labor. The capitalist, however, owns his capital, he owns the right to expect a continuous return long after the capital has actually been paid for. Essentially, the capitalist owns the right to rent his capital to workers: he allows them to survive in exchange for the surplus their labor produces.
Of course, the capitalists do not have absolute political power; the sheer numbers of workers relative to capitalists give them some power to obtain some of their surplus. But it's important to understand that the allocation of surplus is indeed political, it depends on the power of people to promulgate social ideas in others minds and translate those social ideas into legal and political principles that are actually enforced by police, prisons and soldiers. The allocation of surplus is not a matter of "objective" physical law. More precisely, the physical laws that govern the allocation of surpluses are psychological and social; they are not independent of people's minds.
The question is not: what sort of a society must we have. The question is: what sort of a society do we want to have?