#1 The desire to avenge the past
Finally, there is a score to settle, and it is a score that rankles with Mourinho more than Guardiola. During their rivalry in Spain, Barcelona were seen as the purists choice, and Mourinho's sour relationship with the Spanish press did little to improve his popularity.
While Barcelona redefined the way the game is played, Mourinho's Real Madrid were seen as the negative element when the two sides met, and the highly-charged El Clasico games were usually dragged down to Mourinho's level.
Even when Real Madrid stormed to the La Liga title in 2011/12 by breaking a string of points and goalscoring records, it was Pep Guardiola that claimed the Coach of the Year award. He was always the popular figure, and as the press and public increasingly turned against Mourinho, so Guardiola's status was elevated at his expense.
It was a situation that hurt Mourinho's pride, and his desire to restore his status over that of his nemesis will give him the additional drive needed to take his Manchester United side over the line this season.