The midfielder's decision was one of the most controversial of the Arsene Wenger era.
He told arsenal.com: "Many years later and I still think about it," Hleb said. "I don't understand. To this day I still don't understand what happened or why I made the decision.
"Of course, Barcelona in that moment was the best team in the world but I was really happy at Arsenal at the time. I had friends, I enjoyed the coach, I was playing for one of the best teams, the fans, the stadium, everything! The fantastic championship, the Premier League, everything!
"Why I left, I don't understand. Barca, of course, were the best team at the time and everybody wants something to try but really I was 100 per cent happy at Arsenal.
"I cried when I talked to Arsene. We talked in the holiday before I left and he told me he wanted to keep me and I was important. I cried. I didn't know what to do in that moment.
"Of course, Barca is a fantastic team. I won titles and everything but I didn't play as much as I did at Arsenal. This is only my mistake because at the time, a lot of things happened in my private life. I started very nervously [at Barcelona]. Always aggressive. It was only my mistake. "