Justice and the tendency towards good: the role of custom in Hume's theory of moral motivation

Given the importance of sympathetic pleasures within Hume's account of approval and moral motivation, why does Hume think we feel obliged to act justly on those occasions when we know that doing so will benefit nobody? I argue that Hume uses the case of justice as evidence for a key claim regarding all virtues. Hume does not think we approve of token virtuous actions, whether natural or artificial, because they cause or aim to cause happiness in others. It is sufficient for the action to be of a type which has “a tendency to the public good” for us to feel approval of it, and to be motivated to perform it. Once we are aware that just actions tend to cause happiness, we approve of all just actions, even token actions which cause more unhappiness than happiness.