Mister Lonely Heart is a self-styled "Grievance Counselor", a changeling who mimics ponies that his clients have issues with. When Twilight Sparkle pays him a visit, he finds out how painfully neurotic the Princess of Friendship really is.
"Friendship is an island that you retreat to. And you fall on the floor and laugh at all the ninnies who don't have enough brains to have your good taste." --Ray Bradbury