Pursuing the meaning of human greetings

I specialize in Japanese linguistics, sociolinguistics, and the dialectology of the Japanese language. More specifically, I investigate linguistic behavior and greeting expressions, their geography, history, and crosslinguistic aspects.
This inquiry grows out of several mundane interactions during my first university year. In one, a graduate student was leaving early and I suddenly had to say goodbye to my senior in both age and position. Lacking experience, I fumbled for a proper phrase to no avail. My wording ended up terribly awkward.
Then one day I heard another senior student say, ‘Take care!’
and immediately it clicked. I must have heard this expression countless times before, yet never processed it. But on that day, it became a part of my vocabulary. I realized then this must be how people learn ways to address others.
I developed an interest in all kinds of expressions in the following years. My bachelor thesis discussed the paradox of using ‘Good morning’ after midday, a common practice in the entertainment industry. However, my curiosity was not satisfied.
Why do some people use such greetings, while others do not?
Do those who accept using ‘Good morning’ after noon process it in the same way I processed ‘Take care’? What is their logic?
These exciting questions, as well as my fascination with new language phenomena, have driven me to the field of research in greeting expressions.
Some twenty years later, my research also includes inquiries into dialects and the history of language. Furthermore, it now extends far beyond the borders of Japan. But I still remember that first crucial sparkle of wonder, my first step into the world of linguistics.