Discord’s lack of empathy and a pony's sensibility has been tolerated by Celestia, but while both the princesses and the bearers of the Elements of Harmony struggle with even more dire problems, Discord is forced into a lesson of his own.
Twilight panics when she discovers that Princesses are expected to have formal dates for Hearts and Hooves Day, so she travels to Celestia for advice, getting a lot more than either of them bargained for in the process.