There is an impulse in the fallen human—all our hearts—to forget that gratitude is a spontaneous response of joy to receiving something . . . . When we forget this, what happens is that gratitude starts to be misused and distorted as an impulse to pay for the very thing that came to us “gratis” [free]. This terrible moment is the birthplace of the “debtor’s ethic.”
The debtor’s ethic says, “Because you have done something good for me, I feel indebted to do something good for you.” This impulse is not what gratitude was designed to produce. God meant gratitude to be a spontaneous expression of pleasure in the gift and the good will of another. He did not mean it to be an impulse to return favors. If gratitude is twisted into a sense of debt, it gives birth to the debtor’s ethic—and the effect is to nullify grace.
John Piper, Future Grace, p.32