#1 The success of Real Madrid
Ultimately, it came down to this. It wouldn't have hurt as badly if the league and Champions League success had belonged to, say, Atletico Madrid – but it feels bitter for any Barcelona fan that Real Madrid just enjoyed one of the most successful campaigns in their history while Barca was left to look on and applaud.
Real Madrid won their first league title in five years. They won only the third European Cup and league double in their history, the previous ones being in 1956-57 and 1957-58. They became the first team to retain the European Cup since it got rechristened to 'Champions League' in 1992.
It was not just a bad season for Barcelona, it was a historic one for Real – all the more why it hurt as badly as it did.