Of the two sister emotions, regret is by far the more studied. Regret--or, more accurately, our desire to avoid future regret--is a major factor in how we decide what to do, what to say, and what to buy. A man might go on an adventurous vacation because he doesn’t want to regret missing the opportunity, or he might forgo ordering a fifth and then a sixth cocktail because he regretted it the last time he did it.
Disappointment, on the other hand, usually drives us to do not much at all, making it a less fruitful emotion for marketers and professional motivators. Negative emotions like anger or fear give people energy and incentive to act; disappointment does the opposite. That difference is reflected in polling right now that shows a yawning enthusiasm gap between Republicans and Democrats, with the former angry, motivated, and eager to vote, and the latter listless and expected to stay home in the upcoming mid-term elections.