Befriending Strangers - Why do we inflict hurt?

I had a few bad experiences with strangers in the past 36 hours. Once trying to get a table at the bar, once at the hockey game, once walking back from the hockey game. For a moment, I started to lose faith in strangers. To say that is a big deal. Most people in this world are strangers to us.

I wonder, what dictates the way people conduct themselves to those they have little obligations to--people like strangers. What makes people act rudely, harmfully or childishly towards people they have no shared experiences with? Conversely, why do people act as such towards people they DO have shared experiences with? It seems silly. If you DON'T know they, why be a jerk? If you do you know them why be a jerk to jeopardize the relationship? Perhaps a reason to be a jerk is retribution to the way someone has treated you in the past? But if that's the case, that still doesn't explain why to be ill towards somebody else in the first place.

Briefly, there are times when I lose faith in strangers...which is to say losing faith in humanity collectively, I guess. But then, someone will be laughing. Or, a baby walks down the street, toddling along with his parents. There are times when the human spirit triumphs and goodwill towards humanity and the earth triumph over greed. Beauty happens. And then, I remember my faith again.