The Doctor watches Jack as he talks, thinking of all the things he didn't know about this man who'd lived for oh so very long. He knew what it was like, to watch those around you grow old, to lose them. Why had he been so quick to push him away for so long?

"I'm definitely sticking," the Doctor promised, his hearts in every word. "Right to the very end."

When you become a great big head in a jar. The Doctor nearly snorted, but managed to keep his expression bland.