[qohen]

In the specific context of courtesy (or, more specifically, the context of wishing people a good morning), I think this more recent rabbi may have the final word, based on his experience at Auschwitz:

The rabbi, now in his eighties, told me in his gentle voice, 'This is the power of a good-morning greeting. A man must always greet his fellow man.'

The full story is here -- it's short -- go and learn:

http://www.hasidicstories.com/Stories/Of_Our_Times/good_morn...