What should Hummels do, stay at Dortmund his entire career? When a player reaches a certain level, he looks to move to a bigger club. It's just what happens.
It's always happened and it always will. Only a select few will stay with a team for their entire career, why is everyone expecting Hummels to be one of those guys? Writing was on the wall that he'd leave sooner or later. Does it leave a sour taste in certain peoples mouth's that the club he is choosing is Bayern? If so, tough luck. Doesn't bother me personally.