Saying Hello because you know soon you'll have to say goodbye and that knowledge hangs over you like a dark cloud throughout the entire meeting.
...I simply can not say good-bye....in my early 20's I lived in Hawaii & adopted the word "aloha"...which means "til we meet again" (or, we are meeting again)...I have not said the word good bye ever since...good byes break my heart...even if it's just until the next day...for friends, lovers, families...whoever...so, enchantingjade...aloha!
Good Bye word is easy then say Hello!
I think before say Hello to anybody, he or she must be ready to talk to you. But you can say good bye to anybody, no matter he or she is known or unknown to you or want to talk to you or not.