The Argentina striker is thriving this season under Atletico coach Diego Simeone.
Alvarez told Marca: "When I arrived they trusted me, they brought me to Europe from River Plate and made me play many games in which I scored several goals and won trophies. Everything was perfect and if I had stayed there I would have still been happy, but I needed something different.
"Inside me I heard a voice telling me that I needed another challenge, something new. And I felt that Atletico could be the place where I could grow better and find the best version of myself."
He continued: "I want to play all the most important matches, but with City I didn't play in the semi-final or the final of the Champions League and these are the matches I would have liked to play."
On playing for Simeone, Alvarez added: “Cholo spoke to me in the days before the Olympics and convinced me by telling me many good things about the club, not only in the present, but also in the future. And I liked that.
“As soon as I arrived at the airport in Madrid and saw all those people waiting for me, I felt important. And now I’m trying to prove my worth on the pitch.”