This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Rainbow in the Dark
At the end of chapter six, Brownie confesses the truth to Rainbow. This prompts her to get mad at him because he almost died twice for keeping his asthma as a secret. You can probably listen to his heart breaking in a thousand pieces by the end of the chapter.
“Look, I’m sorry, okay? I only wanted to be your friend! I always wanted to find somepony who would stay at my side, who wouldn’t betray me…” He turned his head back at me, tears filling his eyes. “That somepony was you. But all I did was screw everything up.”
“Yes, you really screwed everything up, Brownie… You really did. I need some time alone to think about this…”
I had the urge to slap him- to hit him as hard as I could, but I stopped myself. I wasn’t violent. I closed the door, leaving him and his confessions behind before heading back home to organize my feelings and thoughts about everything he had confessed to me. This was going to take a while.
Brownie suffers a depression during the lenght of the entire chapter, until Mecha manages to cheer him up. This makes him gain new strenght and ask for help to the rest of the Mane Six, who refuse to help him because he hurt Rainbow Dash's feelings, with Applejack dealing the finishing blow to Brownie:
“Ya want to do something right, pardner?” Applejack said, pressing her forehead against me. “Forget her, pack your things and go back to where ya came from. You've done enough damage to her already."