Building on your traffic example, I let people merge in front of me for two reasons:
> If merging were seemless (openings offered where needed and taken when appropriate) traffic would flow better and delays would be reduced.
> When entering slower traffic (especially on highways) I widen the open gap between myself and the vehicle in front. I use this as a buffer for the slow downs, spreading the stop spot into a slow spot, and a slow spot into a faster one – producing a regression to the mean across a longer stretch of the highway.

I view both as mostly selfish acts, the second came from not wanting to shift my automatic as much, and the first benefits me by reducing the chaos and inevitable delay experienced at merger points.
Two goals, one stone. So acting for the better good is a selfish act, but one that asks us to see benefit and cost at a more global scale, dovetailing to your point.

Nice site! Keep ‘em coming!