There are lies amongst friends and families, and they always hit the one who is lied to the hardest. Shining Armor is that victim, and now he wants nothing to do with them. He, especially, wants nothing to do with the mare he married.
Changing a personality is not a trivial process, even when the first step you take is to blast the target with the Elements of Harmony. The risk of a relapse is always present, especially if the original personality was a strong-willed one.
Twilight Sparkle has been living a lie. Everypony told her what a beautiful and accomplished mare she was. Everypony thought she was a responsible adult. They were wrong. She was still just a filly trying to stumble her way through life.
A human finds himself in Equestria under unpleasant circumstances, his reaction, get out of Dodge and start a new life elsewhere. But events which brought him and his treatment in Canterlot are not so easily forgotten by those who did it.