Princess Celestia: Why so glum, my faithful student? Are you not happy that your quest is complete and you can return to your studies in Canterlot?
Twilight Sparkle: That's just it. Just when I learned how wonderful it is to have friends, I have to leave them.Could you tell me what "That' it" in here mean? The expression has a wide range of meaning, and it makes the sentence hard to interpret. Thanks in advance.The line is quoted from http://www.dailymotion.com/video/xmzoaz_my-little-pony-friendship-is-magic-episode-2-friendship-is-magic-part-2_shortfilms at 20:40 The transcription is available in http://mlp.wikia.com/wiki/Transcripts/Friendship_is_Magic,_part_2
PonyFan That's just it. That = what you told me. it = the problem, the point, the issue,
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PonyFanThat's just it.That = what you told me.