Have you ever had certain expectations about something only to be let down because they failed to be met? Such is the definition of the term disappointment. Many times expectations are raised due to other people creating a lot of hype.
It could be a movie (supposedly hilarious), restaurant (supposedly great food), vacation destination (supposedly not that expensive), or even something as mundane as a particular model vacuum cleaner (supposedly sucks up everything, but instead just plain sucks).
How many times have you ever heard several people say how great something was only to be disappointed when you actually tried it?