Spoiler alert if it wasn't obvious.
He was betrayed in the Season 4 finale, and therefor he felt the heartbreak that he had caused upon the Mane 6 themselves. Those lines that he told crying Fluttershy was repeated to him by caged Applejack, and I'm sure that's when he realized the pain that betrayal can cause. When Twilight took Discord back, even after betrayal, he felt honest friendship. If he has the opportunity to betray the ponies again, I'm sure he'll remember Tirek's deception and the very ponies that still valued his friendship.
TL;DR He betrayed, was betrayed, knows how it feels, Princess Derpy for Alicorn 2014