The social scene formed by introverts is about substance over form, the process of interaction is a tool and source of recreation.   A line is crossed in the introvert culture when what is a resource becomes a tyrant, a distraction, a source of white noise.

Many introverts find it onerous just to give daily greetings to people, especially if it’s lots of people.   Their reluctance to observe basic niceties is thought of as rudeness by the larger society, even though there is no intention to be rude. 

The greeting/nicety being gone through is just a rote reflex, a meaningless procedure in the world view of one who looks to substance over form.  It is an annoyance and a pain to waste the powerful tool of language saying the same greeting that is said every day with the same affectation of feeling required to mask the underlying annoyance.

What the loud person does not understand is that THEY are being rude.  They maneuver the introvert in question into a position where they MUST respond or suffer censure from the mass society in which they are involved.  How can a  greeting or good wishes mean anything if it must be given on pain of social shaming and punishment?  The cheerful person demanding the greeting is rude, insensitive, a coercer.  The scripted response/action to any such situation is done for the sake of sociability.  Yet for introverts, who do not define themselves by society, is to be put in an upside down situation.  Society is to serve the individuals within it or else they would be better off alone!  The fundamental purpose is perverted when one becomes used by the tool.
So it is an introvert’s nightmare that one must dance like a clockwork figurine when presented with a stimulus that demands a preset response.

To put it succinctly: take the statement. “15 minutes early is on time.”
The statement contradicts itself, 15 minutes early can’t be early.
Similarly, smiling and greeting every acquaintance one runs into on a busy morning is done out of obligation and fear of punishment. Therefore, niceties can’t be nice.