By "courtesy" I mean making others feel comfortable during interactions they have with you. This has some overlap but is still distinct from "good manners", which to my mind are more formalized and less situation dependent; manners can occasionally be discourteous (e.g. griping at someone for using the wrong fork at dinner).
In my personal life I consider myself a very courteous person. Why do I bother? Two reasons, the first being social convention and the second being economic reasoning. The cost of courtesy is basically zero, and as anyone who's been through microeconomics will tell you, when a good costs nothing you consume as much of it as you can.
So why are some people discourteous? Either they take enjoyment from others' discomfort, or face some cost greater than zero for exercising courtesy. Both reasons seem a bit silly, but the continued existence of discourtesy is pretty good evidence that both do matter in the real world.
Courtesy helps the courteous person (I enjoy social interactions more, and get what I want with less frictions from other people) but it also has obvious positive externalities: both for the person I am courteous towards, and maybe society in general when people are happier and have less strife. Imagine how productivity would increase if everyone in the workplace was kind and genial (but still firm and resolute) towards each other...
With its zero cost and social benefit, there's probably a lot less courtesy in the world than most people want. So why not have the government subsidize courtesy? Maybe each person could have a Yelp score, and strangers could rate their day-to-day demeanor, then collect a check (or a "tax break" if its more palatable) from the state governor on a monthly basis.
This probably seems a bit ridiculous, and I don't actually think courtesy deserves a subsidy. However, the case laid out above is just as (if not more) solid than many of the arguments made for other goods and services the government does decide to subsidize (education, medical care, the fine arts...).
Why does courtesy fail the laugh test and these other subsidies do not?
Social pressure and personal embarrassment are often enough to motivate courteous behavior ("virtue is its own reward"), but arguably those same forces could spur private contribution toward other public goods.
Or, maybe courtesy is just such a personal decision that the government has no capacity to control it -- just like smoking, littering, and bottle recycling -- right?
If someone can give a clear standard which divides courtesy from these popular "public goods" I'd be curious to hear it.