Good Morning, Dammit!

As I got on the elevator this morning, there was a woman already on and she mumbled "good morning" to which I mumbled "hi." (To me, good + morning should never be uttered in the same phrase as it is not true!) So I guess she didn't hear me, cause she said with attitude, "I said good morning!" And I said "I said hi." That just amazes me that people are so f****** rude. They do something to be nice and then get an attitude about it. Am I required to respond loudly and cheerfully if someone mumbles at me? Can someone please explain this, cause sometimes human nature just befuddles me.