This can be summed up in one word: retribution. Give back to others a little of the attention they gave you. When you consider the effort of the other you are repaid and start the chain. The important thing is to make others feel good by valuing them. Who does not need to be valued? Everyone always wants to be noticed and taken into account. When we manage to do that we move sensitivities and therefore gratitude.

By @hojaraskita