The Value of a Kindness

I have learned the man's face behind the counter. I see him a few time a week as I smile and push some coins into his hand; I come for the cheap, foamy coffee. Once, I bring my reusable cup and he lets me pay one dollar for a two dollar sized drink.  He does me a kindness worth a a dollar that causes me to pause momentarily.  Today when I hold out my money, he shakes his head no.
"It is okay" he says.
"But I have to pay you" I reply.
He shakes his head again and I thank him. His small kindness again causes me to pause and think momentarily. Why does this act of kindness mean so much to me?
There exists a concept of 'paying it forward'. This means that if you do something kind for someone, that person will do something kind for another person, and one day the universe will reward you for the kindness you originally did. I find this concept to be an affirmation of a Christian idea in Matthews and  Jewish idea stated by Rabbi Akiva, to do unto others what you have them do unto you (Rabbi Akiva said to love ones neighbor as yourself). There also exists the Jewish reciprocal of this which is to not do unto others what you would not have them do unto you (Rabbi Hillel).
A few months ago I found an I-phone in that same coffee shop. I returned it to a girl whose fleeting face I forgot immediately.  I did a kindness which affected a person whom I may unknowingly walk by in the street, a person whose lingering body heat may leave my seat in the library warm. Maybe in some way this kindness which I have done has brought about another kindness and another kindness and another kindness which causes me to pause momentarily one day in a coffee shop.
We do not do for others or refrain from doing unto others because we expect repayment.  We do it to create a positive impact. We do it to improve ourselves. We do it to create relationships of understanding which bind us to the people around us whose names and faces slip beyond our recognition. And every now and then, we get a free coffee which inspires us to do something for a stranger.