I think being disappointment is just a selfish sin.  When disappointment is caused by sin in others it is just compounded.  Now it is not just their sin, you are now sinning too.

Sin is contagious like a cold but more catastrophic like cancer.  How quietly it sneaks in.  No one's heart is ever broken over a cold but cancer... it's a heart breaker and a life changer.  It stops the world and has your attention, front and center.

We often live in resignation with our own sin,  but the sin of others, that will consume us!  It can become all we think of and begins to define that person and our relationship with them.

Like cancer or even a cold their is no cure for sin, but there is a treatment: The cross and forgiveness.  We need to receive it and give it.

Grace.