A coming paper in the Journal of Marketing addresses that very subject. Building on past research on the role of gratitude in human relationships, it argues that a customer who is made to feel grateful most likely becomes enduringly loyal as a result. Gratitude, as the paper bluntly puts it, can “increase purchase intentions, sales growth and share of wallet.” Robert Palmatier, an associate professor of marketing at the University of Washington and an author of the paper, says that making a customer feel truly grateful toward a business is harder than it might sound. And the hard-wired feelings of reciprocity that can trigger gratitude can just as easily trigger the sense that you’re being treated unfairly.