I like to think of this as having a tremendous ripple effect - when I smile and thank the postman, he smiles at the next person he sees and maybe they're nice to the worker in the drive through at the restaurant, who goes home 1% less frustrated to their family.

Probably a bit overstated in that example, but if you walk around doing it consistently, I think that on average you're still making a small difference, while fighting apathy and politics of hate.