Gratitude as a discipline involves a conscious choice. I can choose to be grateful even when my emotions and feelings are steep and hurt and resentful. It is amazing how many occasions present themselves in which I can choose gratitude instead of a complaint. I can choose to be grateful when I am criticized, even when my heart responds in bitterness. . . . I can choose to listen to the voices that forgive and to look at the faces that smile, even while I still hear words of revenge and see grimaces of hatred.
— Henri J. M. Nouwen, Thanks! How the New Science of Gratitude Can Make You Happier by Robert A. Emmons