#1 Frustrated with the fans
Look somewhere else, Madridistas. The news of Cristiano Ronaldo, the heart-throb of Real Madrid fans being agitated with his own supporters were doing rounds in Spanish publications. Ronaldo was not in the form of his life during the initial stages of 2017-18 season. That not only cost him a promised contract, but also earned him some stick from the fans.
Ronaldo was booed by his very own supporters as the team was not doing what they are expected to do. It is understandable that he might have been saddened by this and felt that if not the club, the fans should back him, no matter what the outcomes of the games are.
He also felt a significant lack of urge from Real Madrid's side to keep him in the club, with only the then manager Zinedine Zidane backing him. Hence a move was obvious.