When I was studying psychology (entry course), a group in my class did an experiment for methodology class: They set up at the entrance to the city library, where there's a steady flow of people going through the doors. They'd go through the doors either 1)"rudely" (buffeting past, letting the door slam on the next person, etc) or 2)"politely" (waiting your turn, giving way, holding the door for the next person). What they found was that the "rude" way didn't really rub off on people, but the "nice" way _did_. People would follow suit and keep the politeness up for a bit.

Not an experiment that holds any water scientifically (and wasn't meant to), but the tenet stuck with me: People WANT to be nice, and doing so _spreads_. I absolutely believe that; people are basically GOOD. We have more potential for that than the opposite. We want to help each other, we want to smile at people.