He had an amazing season at Bremen and Mourinho treated him poorly after his return. Mourinho wasn't planning on playing him and De Bruyne wanted to move to Dortmund, who had just sold Gotze to Bayern. Everything was finalized but then Mourinho vetoed the deal, only to sell to Wolfsburg 6 months later. I'll never forgive Mourinho for screwing Dortmund over like that.
Can't fecking imagine how good Dortmund would've been with De Bruyne, Reus, Lewandowski, Gundogan and Aubamayeng that season
They then had to settle for Mhikitaryan.