If one is just reactively friendly one may find oneself inadvertantly offending other people.  By reactively friendly I mean that one only says Hi when someone else says Hi.  I call being proactively friendly when one takes the initiative and says Hi first.  More than once a person complained to me that "He didn't say Hi".  They never have a good answer when I ask "Well why didn't you say Hi". They are offended when other people aren't proactively friendly without thinking "well I wasn't proactively friendly either".   Sometimes people expect you to do more than say Hi and stop and talk with them.  If one is in a hurry it may be best to say Hi, I'm rushing to catch a train, or I'm late for work in order to explain to them why one is not spending the next hour chit-chatting.