Disappointment is a secondary emotion* that usually stems from sadness. It is a feeling one has when their hopes or expectations for something or someone are net met. There is a sense of loss and hurt, which is why is stems from sadness. Whether you are disappointed that you did not get into Grad School, that your partner was unfaithful, or your parents are self-centered, your hopes/expectations for an experience or a person are unfulfilled. The fantasy that you had about an experience or person are snatached away by reality, and it can be somewhat shocking, but definitely saddening.
Coping Strategies: Verbalize your disappointment regarding the specific expectations/hopes that were not met, along with expressing why the failed hopes make you sad. Expressing these things to yourself through writing or any form of expression can help process the feelings, along with talking about it with a friend or therapist.
-Depending on the situation, you may or may not feel the need to tell the person that you are disappointed/sad by their behavior-- but it is more important to recognize and change your own expectations/hopes of them (or an experience), which is again, saddening, but if you do not
accept reality, you may continue to get disappointed and hurt.
-On the other hand, telling the person may change their behavior, but you should not expect that their behavior will change--which is also sad.
*secondary emotions are your reactionary feeling/s about feeling your primary feeling/s. Primary emotions are those immediate feeling/s to internal or external event