Claim
Manners exist to lower the cost of being near you
The principle is this: an action is good manners if it reduces the discomfort of the people in front of you, and bad manners if it increases it for your own convenience or display.
That is the whole engine. Holding a door, arriving on time, not checking your phone at dinner, warning someone before you call: every one of them transfers a small cost from another person to you. Etiquette is the set of conventions a particular group uses to do that. Manners is the intention behind them. Protocol is the codified version an institution publishes, like a court or an army.
Evidence
Face, in the sense sociologists use it
Erving Goffman described social interaction in the 1950s as a joint project in which everyone works to protect each other from embarrassment. He called the thing being protected face, and the labor of protecting it face-work, in an essay from 1955 and then in The Presentation of Self in Everyday Life in 1959.
Penelope Brown and Stephen Levinson built on this in Politeness, published in 1987, distinguishing two needs everyone has: to be approved of, and to be left alone. Most polite forms in most languages turn out to be doing one of those two things. This is why a polite request is usually longer than an order.
Example
Two men at the same dinner do not know which fork to use. The first asks the host quietly and then uses it. The second waits, watches what the host picks up, and copies.
Both are correct. The third man announces that nobody uses fish knives any more and explains why, which resolves his own discomfort by transferring it to everyone at the table, including the host who put the fish knife there. He knows the most about cutlery and has the worst manners at the table, and the distinction between knowledge and manners is the reason this course spends a day on the principle first.
Claim
The rule beats the rulebook
Conventions are local and they change. Which cheek first, whether to bring wine, how late is late, whether to take your shoes off, when to use a first name: none of these are universal and anyone who tells you otherwise is describing their own country.
What travels is the principle. In an unfamiliar room, you cannot know the convention, and you can always ask yourself which action makes the other person more comfortable. That question has an answer in every culture, which is why it is the only thing worth memorizing today.
Action
Audit your last week
Write down three moments from the past seven days when you made something slightly easier for someone else at a small cost to yourself. Then write down two when you did the reverse: took the call in the room, arrived late without a message, let someone else handle the awkward thing.
Nobody scores well on this and the list is not for anyone else. The point is to see the trade being made at all, because most bad manners are not decisions. They are the absence of one.
Caution
The polite man who is unpleasant to be around
Etiquette turns poisonous the moment it becomes a scoreboard. If you know which glass is yours and you use that to rank the people at the table, you have taken a system for reducing discomfort and turned it into a method for producing it.
The tell is what you do with someone else's mistake. A man with manners covers it: he uses the wrong fork too, he reaches for the same glass, he lets it pass without a flicker. A man with only the rulebook corrects it, or worse, does it silently and enjoys it.
Aside
Whose comfort, and who wrote the rules
The obvious objection to all of this: conventions were mostly written by whoever held power, and plenty of them exist to mark who belongs rather than to make anyone comfortable. Dress codes and table rituals have been used as filters for centuries, and knowing them is partly an inherited advantage.
That is true and it does not resolve into abandoning the conventions, because the person across the table did not write them either and still feels the effect. The workable position is to learn the local forms well enough that they cost you no attention, use them to put others at ease, and never use them as a test. If a rule can only be satisfied by humiliating someone, break it.