When flat-knackers busy it can be debilitatingly irritating getting salutations and farewells from everybody, I'm not being rude (really) when I fail to return the greeting.
When quieter I understand it is just human nature & I will be polite and grunt a "g'day" when greeted but am perfectly happy with those that are more professional and don't. I don't initiate it unless it is somebody I know (and like), and then only if it is real quiet.
If ATC give you an effusive greeting feel free to reply pleasantly with your callsign!