The Swiss international described last season as "very difficult" as he fell out of favour with coach Robin Dutt, and then his replacement at Leverkusen, Sami Hyypia, as well.
The midfielder struggled with a long-term injury that restricted him to just seven league appearances last season which led him to realise that he "needed a change".
Despite being linked with a move to Lazio, Barnetta chose to remain in Germany, and is full of enthusiasm regarding the team.
"The fans are unique, and then there's the sporting challenge of the Champions League. At this club, we want to play at the very top of the Bundesliga."
The 27-year-old also explained why he has been assigned the No.27 shirt at his new club, rather than his usual No.7: "I have not asked for the No.7. It would not be appropriate to compare myself to Raul."
The former Schalke striker was a favourite amongst the team's supporters, despite spending just two years at the club before moving on to Al Sadd this summer.