"The mysterious character of the commodity-form consists therefore simply in the fact that the commodity reflects the social characteristics of men's own labour as objective characteristics of the products of labour themselves, as the socio-natural properties of these things. Hence it also reflects the social relation of the producers to the sum total of labour as a social relation between objects, a relation which exists apart from and outside the producers. Through this substitution, the products of labour become commodities, sensuous things what are at the same time supra-sensible or social."
Marx believes that commodities or products for sale have become desirable to people for their own sake. People want what is for sale because those items are the products of labor. A product is the natural result and reflection of a person's work-ethic, so people like to participate in the economy in order to own something of other people. It's a sort of fetish-like obsession with social relation.
"One thing, however, is clear: nature does not produce on the one hand owners of money or commodities, and on the other hand men possessing nothing but their own labour-power. This relation has no basis in natural history, nor does it have a social basis common to all periods of human history. It is clearly the result of a past historical development, the product of many economic revolutions, of the extinction of a whole series of older formations of social production."
Marx does not believe in the dominant system of economics -- capitalism -- because he does not see it reflected in nature. According to him, he cannot observe two separate classes of human being born, one to own production and the other to be the slave of production for the owner. This is what he believes capitalism promotes -- the belief that some individuals must serve the greater economy by working for somebody else in order to a) provide for their own needs and b) to line the pockets of their masters.
"In other words, the labour of the private individual manifests itself as an element of the total labour of society only through the relations which the act of exchange establishes between the products, and, through their mediation, between the producers. To the producers, therefore, the social relations between their private labour appear as what they are, i.e. they do not appear as direct social relations between persons in their work, but rather as material relations between persons and social relations between things."
In this excerpt, Marx explains how people view a commodity. They see a line of products on a shelf and do not usually comprehend the social relationship between manufacturers and laborers which made the products possible. Most people see the relationship between the value of the products themselves without a thought of the social consequences of their existence, i.e. the people who made them.
"Labour, then, as the creator of use-values, as useful labour, is a condition of human existence which is independent of all forms of society; it is an eternal natural necessity which mediates the metabolism between man and nature, and therefore human life itself."
Essentially, Marx believes that labor is a common necessity across all humanity. The nature world relates to humanity through work, the means by which we invent value for ourselves. Labor is a universally natural condition which applies to all of humanity.
