I think smiling at strangers are a cool (albeit very brave) way of breaking the monotony of city life. Better to be safe than sorry, I know, but sometimes I get fed up with all the mistrust I feel with people walking at top speed, eyes downcast, short of taping their belongings to themselves, lol.
P.S. When I get out of the office and Im feeling “mean”, I usually nod at complete strangers on the way home. You can really see how unexpected it is for them!