Given interdependence of each on others, many, probably most, of whom we are unlikely ever to know, it is well that we can benefit from their actions without it being necessary for the happy bonds of ‘mutual love and affection’ to exist among us. We only ever know a few of our relatives; cousins twice removed are usually strangers of whom we may know nothing at all. Our circle of friends is also limited, and while our acquaintances are a larger set of people, they are still dwarfed in numbers by the billions on the planet (even the millions in our respective countries’ or even those who live in the same town).
When Smith was teaching his classes, using materials that appeared in Wealth of Nations 17 years later, he focused on this factor in human life and explained how people could be in complex exchange links many intersections long and could still benefit from countless transactions among millions of strangers with whom feelings of ‘mutual love and affection’ were not realistic nor necessary. The key was the 'mercenary exchange of good offices' (TMS), or market-based exchanges, as elaborated in Wealth Of Nations.