Rose Tyler: 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?
Doctor: No. Not to you.
Rose Tyler: But Sarah Jane. You were that close to her once, and now, you never even mention her. Why not?
Doctor: I don’t age. I regenerate. But humans decay. You wither and you die. Imagine watching that happen to someone that you…
Rose Tyler: What, Doctor?
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.
From Doctor Who – Season 2 Episode 3: ‘School Reunion’ (2×03) | Produced by BBC