You've rehearsed it over and over in your mind. You think about grammar and spelling, how you will address every issue, every hurt. You write it out as a letter/email.
And then at the last minute you ask a good friend "So what do you think?" and she responds "I'm glad you wrote this. I hope it helped you to address everything that was bothering you. My suggestion is, sleep on it."
That's the last thing you want to hear. You want to send it; you want your friend's approval. You can't wait to tell this person how you feel.
But, you decide to sleep on it. You think about something else your friend said: "Years from now, will it matter? Will it change the person? Could it cause harm?"
You decide to listen to your friend; you don't send the letter. Several years pass and you find your hand written notes. And you can't even remember what you were so angry about to begin with.