I think what good manners comes down to, from a purely psychological standpoint, is behaviour specifically designed to maintain the correct vagal tone with your audience. Dressing properly, being cooing and gentle, showing up at a party at the correct time, dealing with disputes in a correct manner, etc. all come down to (as many before emphasized) making others as comfortable as possible. Comfort has its own associations with cognitive accessibility, a smoothly running system one that is socially engaged and not in a splitting/defensive orientation.
It's opposite, rudeness, seems to be like a disrupting of that tonus, which naturally causes us to revert to a more fight-or-flight behaviour (calling somebody rude is itself saying this person is breaking boundaries and not considering others). Usually if it is a severe enough disruption, those who also call out someone for being rude, while chastising them, aren't considered rude because they did not contribute to the original disruption. They also validate the feelings of those who feel they are being assaulted by a lack of consideration, and in effect help restore the emotional solidarity felt prior to the rude interruption. This could be why Laura didn't feel bad for putting the righteous verbal hurt on the imbecilic auditor.
From this perspective, what constitutes good manners can vary between social groups. In some more sardonic friend circles I know, sarcasm and subtly insulting one another is just a form of play. If you were to derail those types of comments with the naive politeness you're used to showcasing around the elder's home, you would probably get awkward side-glances from people. So I often do need to give mixed company a cold reading to try and determine the types of behaviour they consider the least threatening, most welcoming, et cetera.
Personally though, as much as I try and keep people humming along hunky-dory with my good manners and such, a part of me wishes that people were less sensitive to disruptions of their vagal nerve stimulation from inconsequential garbage (e.g., if arriving at a party without a frock coat is enough to send guests into a splitting frenzy and thinking him/her rude, you really have to wonder about the stability of the rest of their instinctive substratum!)
But codifying good manners into all these behaviour rules as was the fashion in Victorian times seems more to me a trait of more hysterical cultures. OSIT. By their standards, Diogenes of Sinope was a pretty rude guy, wasn't he?
But he lived a more natural life than many contemporary Hellenics, and I'm sure a hunter-gatherer would find his behaviour far less rude than a regular Athenian.