Disappointment is the feeling of dissatisfaction that follows the failure of expectations or hopes to manifest. Similar to regret, it differs in that a person who feels regret focuses primarily on the personal choices that contributed to a poor outcome, while a person feeling disappointment focuses on the outcome itself. It is a source of psychological stress.
The study of disappointment—its causes, impact, and the degree to which individual decisions are motivated by a desire to avoid it—is a focus in the field of decision analysis, as disappointment is, along with regret, one of two primary emotions involved in decision-making.
The feeling of disappointment is a adjustment, or reconciliation between fantasy and reality. Disappointment can only occur when your expectation of how things should be is different from how things really are.
Disappointment is not harmful but it indicates that you may have a tendency to create and attach to fantasies. Use it as a means of auditing and adjusting for that tendency.