"The great secret of morals is love; or a going out of our nature, and an identification of ourselves with the beautiful which exists in thought, action, or person, not our own. A man, to be greatly good, must imagine intensely and comprehensively; he must put himself in the place of another and of many others; the pains and pleasure of his species must become his own. The great instrument of moral good is the imagination..." - Percy Bysshe Shelley, "A Defense of Poetry"
Update: Just remembered that I have already quoted this before. I'm also partial to Robert Green Ingersoll's expression of a nearly identical sentiment.