It took the Doctor a while to realize that he did, in fact, have an obsession with hats.

Not that that was a problem, of course not. After all, fezzes are cool, as are Stetsons. But if there was a problem with this revelation, it was that it made the Doctor think about Donna, how she had come onto the TARDIS prepared for a trip to the planet of hats. He thought back to the hatbox, and realized that it must still be in the wardrobe room.

As he went through the box of hats (maybe there will be a fez in here somewhere...), the Doctor also thought about Rose, and how he sometimes thought that his last form had been made with Rose in mind. He wondered for a brief moment if perhaps this form, with its penchant for hats, had been made with Donna in mind, even though he had already let her go by the time he regenerated.