The 28-year-old, who joined the club from Manchester City in 2008, has scored 63 times in 218 appearances for the Hoops.
However, the forward has just one year remaining on his current deal, with Celtic yet to begin talks over a new contract.
"I enjoy my football here. I'm at a club where I feel well. It was important to me to find an environment where I feel good because I'm away from my country, my family and friends," he told Sky Sports News.
"I need an environment that suits me and here I am. I'm not surprised how long I've been here. I always felt I'd be here a long time. I feel great at this club and that means everything to a footballer.
"Celtic made a lot of sacrifices for me and I did for them also. We have an understanding relationship and that's why I have been here for nearly six years. I have a great relationship with the manager and, if he wants me, I want him as well."
Celtic face Cliftonville in a Champions League qualifier on Wednesday.