From the moment he laid eyes on her, the Doctor could swear he'd seen her somewhere before. With her brown-rooted blonde hair and a luminous, wide smile that rivaled all the distant galaxies, she infected him. Everywhere, he saw the phrase, "Bad Wolf," and knew it was somehow her.
Then, when the TARDIS stole her away to safety and Rose Tyler became the closest thing to a human Time Lord the universe had ever seen, the Doctor remembered. The light of time itself shone through her eyes and the Doctor's most despised Moment came back to greet him like an old friend.
He should have heard their screams. Those tired, old ears remembered them so well, the screams of every child as he destroyed them and their only hope. They remembered the whisper of a voice so tired and lifeless, so lost, it sounded as only a hoarse cry.
"No more."
Instead, he heard the voice that saved a Dalek. He saw his Rose Tyler, but knew it wasn't her. Not entirely. The disobedient human child who had a knack for needing saving had returned to save the Doctor. She had torn through all of Time And Relative Dimensions In Space to get there and bring the Big Bad Wolf down on the Dalek fortress.
It was impossible for her to survive, the Doctor knew. But he couldn't help staring in wonder as his companion, his Rose by another name, destroyed the Dalek fleet with a mere thought. This Bad Wolf was beautiful and powerful, but nowhere near as sweet as the human girl who brought her wits with her no matter where they went. He would not let her wilt and wither to make room for the TARDIS incarnate.
He took her Death inside him as only a Time Lord could and held it at bay as long as he could. Long enough to tell her, in case she didn't know.
"Oh, Bad Wolf Girl, I could kiss you!"
Yeah, that does happen.