People often say, "Thank you," even when it is unnecessary. Even in places where goods are being fairly traded, value for value, we often say, "Thank you." Why? One reason is that there are often little things people do beyond the demands of the bare transactions which benefit us - like a smile or an encouraging word or a gentle and caring demeanor. These we sense as unpaid for grace and so we feel gratitude in our heart for them.
But there is another reason we say thanks when people are merely doing for us what we'ved paid for. Since gratitude is universally known as a feeling that comes in response to grace, expressions of gratitude have come to be used as expressions of humility and encouragement. When we say, "Thank you," to someone, we humble ourselves as a person who has needs, and we exalt them as one who can meet those needs.
For example, in a restaurant why do I say, "Thank you," to the waitress for bringing my meal? I will pay for it, and I will tip her. She is doing nothing beyond her duty. The reason is that "thank you" is a gesture of humility that says to her: I am not eager to exalt myself as one to whom you owe service. I do not wish to presume upon your work as my due. I am happy to put myself in the position of one whom receives grace. I am happy to honor you as one who can meet my need.
The reason a simple unnecessary "thank you" can say all of this is because of its close association with grace. Grace begins when one person is full and another is empty. When the grace of Jesus penetrates the human heart it rebounds back to God as gratitude.
- John Piper