The word disappointment originates from the Old French word désappointer which means “to deprive of an office or position.” In relationships, when we feel disappointed in a person - we are feeling we have been deprived of the potential (the hope) we had in them and for them. This is the grief of disappointment. 

We had hope in that person, and they did not live up to our idea of who they should/can be. Is it about them, or is it about YOU?