The Ben Franklin effect: ask someone for a favour to make them like you more

Benjamin Franklin noticed that a political rival who had once lent him a rare book became friendlier towards him afterwards. Psychologists later confirmed it: when we do someone a favour, we unconsciously justify that behaviour by deciding we must like them. Asking for a small favour — rather than doing one — triggers this self-justification loop in the other person. The paradox is that making yourself slightly vulnerable or dependent increases warmth toward you, not resentment. Franklin weaponised it deliberately throughout his political career.

Source: Jecker, J. & Landy, D., 'Liking a Person as a Function of Doing Him a Favour', Human Relations (1969)