Good Whatever

Allow me to share something that keeps me up at night. You know, other than real problems:

Being the dorky English teacher/major that I am, it's a word question, dealing specifically with the word "good." No, it's not the good/well debate, though thats a fun one, but the "goods" that precede times of day.

For example: good morning, good day, good evening, good night. They're all greetings, which makes sense. Here's what bothers me, though: good morning and good evening are ways of saying hello, while good day and good night are ways of saying good-bye.

If English made sense, which it doesn't, which is why it is the most difficult subject to study/teach of them all (ha), then they would all be meant to either welcome or dismiss people. Is there some rule or logic that I am not aware of?

I've tried to figure it out. Saying "good day" or "good night" is shorthand for "have a good..." but the same does not appear to be true of "good morning/evening." The latter pair are earlier times, so I guess it would be more likely you'd be saying hello at these times, but not always. What about sluts? (I ask myself this question a lot.) After crawling out of somebody's bed early in the a.m., they can't just say "good morning" as that would imply they're sticking around, which please.

Now I'm too confused to even figure out how to sign off this post. I'll just go with a standard "good"-y: good-bye.