Necessities of Niceties

Why are niceties so important ?

example: i  get down from the train, see someone walking in front of me whom i know...and i dont want to meet them....except that i am sure that the person will see me when he/she looks up/turns around...why am i expected to greet them and smile and make nice, especially when i know for sure there is no future chance of me making dirty with them??

blehh and more to come....