From the way I understand it, one can only disappoint you only by your expectations. That is, If you are involved with someone you know is a playboy, and you expect that they won't settle down, you know that they will not fall in love with you and you agree that the relationship should just be casual, when that person leaves of has another you would already have expected that.
On the other hand, the same type of guy, you believe he has changed, you think he will settle down and that the relationship is serious. When he cheats or leaves you are hurt.
So the same guy, tow different expectations, two different results.
Another example is a person who has been to jail. You know the guy is a thief so when he goes to jail you are not surprised, but if he leads you to believe he has changed or you don't know about his past, when he goes to jail you are shocked. It's you perception, belief or expectation that has been shattered.
In conclusion, when you love someone, you have certain expectations and beliefs. When they go against those beliefs and expectations that's where it gets painful. In most cases they are the ones who lie to us and make us believe in them and expect the most.
At least that is what my Psychologist friend says and I am apt to believe it based on personal experiences.