Klaassen turned down a move to Turkey in January before signing for Werder last summer.
He told BILD: "I just feel comfortable here, a lot of things work out on their own. Werder's style of playing fits in much better with me than with Everton. I am relieved that everything is going so well here.
"We have great boys, a good support staff, a good coach. We all hold together. Characteristic of this was also the speech of Nuri Sahin before the game against Wolfsburg. That has pushed us extremely."