According to economic theory, there will be a certain level of "supply" and "demand" for a given product in an economic system.
It is up to this principle that individuals engage in activities in order to provide different types of goods according to the level of demand each of them has. If there is a growing demand for a certain product, individuals within the economic system will probably make efforts in order to produce goods that will supply that demand and make them profit. Demand will stabilize when the price for a determined good represents the maximum amount a buyer is willing to pay for it.