The DoctorDonna || Tentoo and Donna
The concept of Donna Noble, not knowing who he was, made the Doctor sad.
After all, this human Doctor owed everything to her. He wouldn't exist without her. They were connected in a way that superseded anything else; she'd been his best friend, and had given him something he'd always wanted but could never have. One life. One, simple, human life, that he'd been able to spend with the woman he loved. She'd always said that him being with Rose was good. Always supported it, supported him and her, even if he knew that he couldn't. Because of her, he could.
Maybe that's what'd motivated him to be so honest with her. That they'd been best friends. That he'd trusted her more than anyone. His single, beating heart was because of Donna Noble, and there was no way the human Doctor was going to allow her to see down upon herself when she'd done so very much. When she'd saved the universe. Saved the Doctor.
He had Torchwood training (though there were moments, he'd admit, he disagreed with their methods). And though they were the ones who'd sent he and Rose there on word from word of a very reliable source, he was more than willing to push a few of their limits and break a few of their rules. It was the irrationality that came with the human in him.
So that's what brought him outside H. C. Clements, pacing awkwardly in his favorite blue suit and trainers, hoping that something about him in the familiar clothes could bring a memory or two of him back to her. Donna'd always been by his side. And now, he needed her more than ever.