There is a difference between expressing thanks for something and being genuinely grateful for it. For example, let's suppose you are a craftsman and you agree to do a certain amount of work for so much money, and you fulfill your obligation. The work is done within the prescribed period of time and you do it well. When you fulfill your contract you are paid. Common courtesy demands that you respond, saying, "Thank you!" as you pick up the check in the brown manila envelope and take your tools home.