I think the only satisfying answer is in Marx's mature social theory. He thinks that the fundamental thing which differentiates capital from previous social forms is that capital involves a historically specific form of wealth, which he calls value.
In capitalism, not only are there markets, but the exchange that happens in capitalist markets depends on the principle that the thing I am exchanging has (or rather "is") both a "use-value" and an "exchange-value," which are invisibly in contradiction with one another. Its value in exchange is determined by the average socially necessary labor time required for its production, whereas its use-value has to do with its physical qualities. In capitalist society, absolutely everything has an exchange-value and therefore a price, so absolutely everything can be exchanged for.
The result is that -- whereas previous forms of society might distinguish between, on the one hand, things that existed to be admired, to mediate relations between people, to indicate status, (often these are the things you can't buy or sell) and on the other hand things that existed to be used, (which you could) -- capitalism makes this distinction untenable. Every use turns out to itself be a mediation and every mediation a kind of use.
According to Marx this has immense consequences, and his book Capital is basically an attempt to show that it's ultimately because of this double character of commodities (the coexistence of use- and exchange-value) that capital is (or rather "has") a dynamic/logic. Very simply, this is because though people think of themselves as "using" things in capitalism, the "reason" things are made is because their value in exchange is the way invested capital is realized. Firms are compelled to revolutionize their production process making it more efficient by reducing the amount of labor required, which temporarily makes them more profits -- only the long term result is that other firms must do the same, ultimately lowering the rate of profit. (Since, as we said, exchange-value is a function of average socially necessary labor time). Ultimately, this means that capital is constantly searching for new markets in which it can realize itself, "breaking its local bonds," as Marx puts it.
So capitalism is "dynamic" in the sense that it's compelled by its own logic to blindly expand in every direction, not to get more stuff, as older societies sometimes did, but to get rid of it, by opening new markets that might match the absurd excess of commodities it is constantly producing.