Gotze will officially become a Bayern player on July 1 after the European champions completed the 37million euros signing of the Germany midfielder - a deal confirmed by the Bavarian club on the eve of Dortmund's Champions League semi-final against Real Madrid.
The timing of the announcement was frustrating to Dortmund, but Hummels was even more "annoyed" by its content.
"Everybody could see how good our team is," he said. "I just don't think there was or is any reason in sporting terms to leave us.
"You could see how our team had developed and Mario got on so well with many, which is why it annoyed me so much that he made up his mind that he had to leave so soon."
Hummels, who made the move in the opposite direction three years ago and has also been linked with a return to Bayern, has told Gotze how he feels.
"It came as a surprise to all of us in the team," he told Sport Bild magazine. "I have told him that."