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Tear Jerker: Friendship Is Empathy
The argument between Rainbow Dash and Twilight is just heartbreaking, from Dash revealing how hurt she was by Twilight and the others' plot to humble her, to Twilight's nasty snarkiness after Dash pushed her berserk button (the Want-It-Need-It spell catastrophe). Ultimately, they are both very sympathetic, as while Rainbow was being more egotistical than usual, and Twilight did come off a little crazy accusing Princess "Cadence" of being evil, they were both extremely hurt by how their friends reacted.
When Applejack finds a tearful RD and tries to comfort her, saying that hurting her was never their intention, Dash responds: "But you did."
Later when AJ and the other three try to speak with Twilight at the library, she rejects them because she thinks they would just think it was her going crazy as usual.
When Applejack asks Rainbow why she never told them about how hurt she was over the Mare Do Well incident, the answer is truly heartbreaking.
Rainbow Dash: “You went behind my back and put on a full-blown costume just to humiliate me. I…I can’t trust you guys anymore. Not like I could before. I thought you’d just think that I hadn’t learned my lesson…I didn’t think I could take another lesson.”