Disappointment

It is an absolute truth that everyone will disappoint you sometimes.  

Let's just sit with that for a minute.  Your mother,  your father,  at least once in your life,  they'll get it wrong,  and let you down.  Your sister,  your lover.  Won't hear, won't be there,  will let you down.  Hell,  even you,  yourself,  may disappoint yourself sometime...  I shouldn't have done that,  drunk that,  slept with him...

So everyone will disappoint you.  

But some people are worth it.  

It helps not to expect or even desire perfection.  If they meet you half way,  if they don't want to disappoint you,  but life got in the way,  if they are trying.  And if on the good days,  you both laugh and laugh.  Or if they remember your secrets and never share them, or if they always speak their truth to you and let you speak your truth to them... then you can weather some disappointment.

(of course I'm not talking about major betrayals here. They get a lot more scrutiny and thought...)