Gotze, 21, will begin next season with Bayern after agreeing terms with the Champions League winners, but Hummels has admitted that he is struggling to understand why the German would want to leave Dortmund.
"I do not understand Gotze's decision and I have told him just that. Everybody at Dortmund was surprised. This only further shows that this was never an issue," Hummels told Sport Bild.
"Everybody can see how good our team has become. I simply do not believe that there were any sporting reasons to leave BVB.
"It was clear to see that the team had really made steps forward and Mario got along with all of us very well. That's why it annoyed me so much that he felt it was necessary to leave at such an early stage of his career."
Gotze is a product of Dortmund's youth academy.