Sunset Shimmer knew she was better than the other ponies around her, even if she didn't have her cutie mark yet. She had to be. She just needed to find a way to prove it.
It was a simple bet. Spend one month as a pony, and prove to Celestia that he had been right all along. Discord never thought he would actually enjoy it. As the days pass, he finds himself desperate to keep up the façade a little longer.
Discord's made many decisions in life he's not proud of, casting that spell on Twilight being one of them. Left without his magic, he must rely on Twilight. In her, he finds a confidant and the courage to bury his past failings at last.