There was a time when BVB fans were furious with Lewandowski. He made it clear that he wanted to leave for Bayern Munich and while the club wouldn't sell, he moved quickly to sign a pre-contract with the Bavarians in January so he could join them when his Dortmund contract expired this summer. It was always going to hurt Dortmund to lose their star striker, but to lose him to Bayern, of all teams, that was especially painful.
Lewandowski's desire to leave made sense. BVB doesn't pay players nearly as much as Bayern and playing for Bayern is almost a guarantee to play for a Champions League contender. Dortmund may compete in some years, but they won't be among Europe's elite year in, year out.
Eventually, BVB fans came to terms with Lewandowski's departure, and they sent him out wonderfully.