Eftersom hon kommer att dö och han kommer inte.
Från Skolmöte :
ROSE: But Sarah Jane... you were that close to her once, and now... you never even mention her. Why not?
THE DOCTOR: I don't age. I regenerate. But humans decay. You wither and you die. Imagine watching that happen to someone who you...
He stops when he realizes what he was about to say.
ROSE: What, Doctor? The Doctor stares at her intensely, as if willing her to understand.
THE DOCTOR: You can spend the rest of your life with me.
Rose looks up at him, eyes shining with unshed tears.
THE DOCTOR (CONT'D): But I can't spend the rest of mine with you. I have to live on. Alone. That's the curse of the Time Lords.
Läkaren vet att han sannolikt kommer att överleva Rose, och han kan inte göra det för att uppleva det.
I Journey's End ger han "henne" sin klon specifikt för att han kommer att ha samma livstid som henne.
ROSE: Hold on, this is the parallel universe, right?
DOCTOR: You're back home.
DONNA: And the walls of the world are closing again, now that the Reality Bomb never happened. It's dimensional retroclosure. See, I really get that stuff now.
ROSE: No, but I spent all that time trying to find you. I'm not going back now.
DOCTOR: But you've got to. Because we saved the universe, but at a cost. And the cost is him. He destroyed the Daleks. He committed genocide. He's too dangerous to be left on his own.
NEW DOCTOR: You made me.
DOCTOR: Exactly. You were born in battle, full of blood and anger and revenge. Remind you of someone? That's me, when we first met. And you made me better. Now you can do the same for him.
ROSE: But he's not you.
DOCTOR: He needs you. That's very me.
DONNA: But it's better than that, though. Don't you see what he's trying to give you? Tell her. Go on.
NEW DOCTOR: I look like him and I think like him. Same memories, same thoughts, same everything. Except I've only got one heart.
ROSE: Which means?
NEW DOCTOR: I'm part human. Specifically, the aging part. I'll grow old and never regenerate. I've only got one life, Rose Tyler. I could spend it with you, if you want.
ROSE: You'll grow old at the same time as me?
NEW DOCTOR: Together.