Riise said: "Clubs like Liverpool are always looking round the world for players, and there is always going to be someone new challenging you.
"It was sudden. One day I was a Liverpool player, the next I was going to leave.
"I just wanted to make sure I got the best possible club. There was no way I was going to be comfortable playing for another English club, and I had plenty of offers from other clubs in the Premier League.
"I didn't have to leave because I had another year left, but I did want to be playing regularly and I didn't feel that would happen this season if I stayed.
"I spent seven years at Liverpool and played more than 250 games. The challenge in Italy was the right one for my career and the time was right to change to another country."