Good manners

From Galateo: or, A treatise on politeness and delicacy of manners: some unexpected advice.

For the same reason, it is by no means a decent custom for any one, upon meeting with any thing offensive in the way, (as it often happens) to turn immediately to his companion, and point it out to his notice: much less ought he to hold up any thing foetid to another, that he may smell to it; which some people are apt to do; and are even so impertinent as to thrust what is nasty up to their very noses, and smear them with it: "Pray smell it, I beseech you, how it stinks."

On the other hand, some things don't change much:

It is also very impolite to appear melancholy and thoughtful; and, as it were, absent from the company where you are, and wrapt up in your own reflections. And, though this perhaps this may be allowable in those, who, for many years, have been entirely immersed in the study and contemplation of the liberal arts and sciences(*): yet in other people, this is by no means to be tolerated. Nay, such persons would act but prudently, if, at those seasons when they are disposed to indulge their own private meditations, they would sequester themselves entirely from the company of other people.

(*) Thomas Aquinas, dining with the king of France, after a short pause, with his eyes fixed, struck his hand upon the table, crying out; "I have confuted the Manichaeans."

Or

But, however this may be, we ought not to bring a gloom over the minds of those with whom we converse, especially in places where people meet together to enjoy themselves, and not to lament the miseries of human life: although, perhaps, we may sometimes meet with a gloomy mortal of weak nerves, who is fond of squeezing out a tear upon all occasions; whose longing one might easily satisfy by the acrimony of a little mustard, or by entertaining him in a smoaky room. ... To introduce a narration, therefore, of such dismal and melancholy events, on such an occasion, is so very absurd that it were much better entirely to hold one's tongue.

This one struck a little chord:

When, therefore, you address a single person of any rank, who represents a number of people as a society, you do not pay him that civility on his own account: and, if you should speak to him in the singular number, (and call him thou instead of you) you would deprive him of what was really his due, and certainly affront him, by giving him an appellation which belongs only to mere rustics, and men of no importance.

In A Secular Age Taylor notes that there was a change in the meaning of etiquette and good manners some centuries past, in which one was to treat peers, and even inferiors, with courtesies due superiors. I suppose that's the reason we use you universally, with family and rulers alike. (And y'all if we need a plural.) I often run into people who think Thou is a sacred word used only for God. The font of knowledge cites Webster saying that thou had vanished in southern England by about 1650. (Similarly in Dutch?)

I was told to be quite careful about vous and tu usage, but I'm told that tu/toi is getting to be fairly universal. I wonder if there is, or once was before the meaning was forgotten, a difference in attitude between addressing men equally ("When Adam delved") by being formal/respectful, or by being familiar.