Sarah Jane Smith: I thought you died. I waited for you and you didn't come back and I thought you must have died
The Doctor: I lived. Everyone else died.
Sarah Jane Smith: What do you mean?
The Doctor: Everyone died, Sarah.
And later on:
Rose Tyler: I've been to the year five million, but this, this is really seeing the future – you just leave us behind! Is that what you're going to do to me?
The Doctor: No. Not you.
Rose Tyler: 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 you...
Rose Tyler: 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 Timelords.
Sarah Jane Smith: No. The universe has to move forward. Pain and loss, they define us as much as happiness or love. Whether it's a world, or a relationship... Everything has its time. And everything ends.
I regret that I never had the chance to meet her.