Hahaha.. well, maybe at first, Discord just wanted to have fun a bit with the six, so he chose to stay..
But along the way, Fluttershy gave him things he never had before; a trust and a friend.. Fluttershy believed him with all her heart, she promised him not to use her element against him and tossed her necklace.. not even Celestia would give him that kind of trust.
And so, embraced by the true kindness, even the Mighty Discord wouldn't want to betray the feeling.. And then, you know the rest.. all is well that ends well..! Ahahaha..!
I'm still thinking that was too fast, for someone who almost destroy everything in the Begining of the 2º Season... controling easely the sun and the moon, and the earth itself, and now, all of the sudden, he was changed... I Spected at least, some Harder resistance, too hastily the End... They've could done the 2º Episode.
Discord is too great to be changed all of the Sudden by a Single Act, too hastily for someone like him.
OR... maybe they're preapering a new villain, worst than Discord.
Well, to be honest, I have a little faith in Twilight.. because.. let's face it: Twilight is helpless without books.. whenever a problem occurred, Twilight will be rushing to her library and dive into a pool of books.. And I.. was a little bit disappointed by how she reacted towards Trixie earlier ago..
So as for now I, too, agree with Celestia, or the writer, who put Fluttershy in charge to reform Discord.. After all, she is a wild beast tamer! Ahahaha..!
Sadly yes, but you know, this could've been yet another test for her. Like when she had to bring back the Crystal kingdom. I know its not any different, but it'd be a good test to take if Celestia wanted to give it to her. Just sayin'.. And if Celestia DID choose Twilight, she could've told her something like, "This time I want you to not look to your books. I want you to use what you learned and know about friendship and use it to help reform him." And Twilight, being the faithful student that she is, would listen.
But oh well. It didnt happen xD Yayyy Fluttershy! And that she is But such a sweetheart at the same time <3 Lol Blah.. here come the Fluttercorders~