Only those who matter to you can disappoint you
This one may seem counter-intuitive as there are a lot of things that disappoint us which we don’t attribute to people, even though it is related to people. For example, two sports teams you might not care a lot for, but you would have preferred one to win a match and they didn’t. You might frame it as you were disappointed the other team won, then say they didn’t matter to you yet they disappointed you. Well, if you analyze it, the team you cared for lost, even in the slightest use of the word “cared”.
If you had any feelings about the matter, then it mattered to you.
Counter intuition and interpretation aside, the point of this quote is to realize how you truly feel about someone. Perhaps you need to learn not to care about them so they can’t disappoint you (letting go). Or rationalize your way through it to realize you have no good reason to be disappointed cause you don’t care about that person. Or maybe realize that person still matters to you cause obviously you’re disappointed in them, so you maybe should decide what to do about that, whether unattaching yourself emotionally or reaffirming the attachment.