Rose: I thought you and me were- Well, I obviously got it wrong. I've been to the year 5 billion, right, but this... Now, this is really seeing the future. You just leave us behind. Is that what you're gonna do to me?
The Doctor: No. Not to you.
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 that you l-
Rose: What, Doctor?
The Doctor: You can spend the rest of your life with me... 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.
~Rose and The Doctor, Doctor Who - School Reunion