In The Wealth of Nations, first published in 1776 as An Inquiry Into the Nature and Causes of the Wealth of Nations, Adam Smith argues that though autonomous in principle, the individual relies on others for the necessities of life. These necessities cannot be obtained solely through the benevolence of others. One must secure them through some form of exchange by appealing to the self-interest (or self-love) of others. Put differently, in order to obtain a good, one must offer something useful in exchange. The freedom to produce in accordance with one's self-interest ensures the availability of goods in a society, both in terms of quantity and variety.
Monopolies and, to a lesser degree, restrictions on employment in certain fields, lead to high prices, which makes goods less accessible to individuals. By contrast, free trade and free competition establish lower prices; they also ensure that enough will be produced but not too much. If at any given moment, there is a dearth of a certain good, individuals will make up for that lack so as to benefit from the void in the market. If for a moment too much is produced, prices fall below the natural (or free market) rate, and production diminishes accordingly.
In other words, free competition and free trade establish a healthy balance of sorts, regulating the quantity of production in accordance with need, and bringing prices to a lower and fair level. Smith argues that this principle extends to all economic spheres: the freer and more general the competition, the more the public benefits, not only in the price of goods but also in the quality of services. Thus, in pursuing their own interests under a system of free trade and free competition, individuals end up benefiting the public.
Smith's The Wealth of Nations can be read in its entirety at the link provided below.