She was dying.
All he could do was sit and watch it happen.
He once told her that she could spend the rest of her life with him, but he couldn't spend the rest of his with her. And now she was fading fast.
He would soon be without her.
But it hurt too much.
So he made an agreement with the Tardis. They had lived long lives. They were both very tired.
So one last trip.
To the very end.
He held her as they floated right on the brink, making her promises of now and forever until the end of time.
Time, matter and all existence fell away in the blink of an eye. And he would never have to live without her.