He could imagine her like that. He'd never thought she'd be a mother, but she fit the role so beautifully. She knew how to be young, and knew how to stay young even as she aged.

"I remember the first time I took her to a different world," the Doctor said. "All young with...with wide eyes and excitement. I felt young."

He'd been in a much older body, then. A sadder, older self. She made him young. She took care of him when he regenerated, she never gave up on him.

"She was my best friend," he said.

He shoved back the part of his mind that wanted to add in and you left her.