"But Sarah Jane... you were that close to her once, and now... you never even mention her. Why not?" Because it hurts. Because I never thought I'd see her again. Because she is one of the few I've known who isn't dead. Because the past brings back memories I don't want to see. Because--
"I don't age. I regenerate." You've seen it yourself. It's still me but different. I act different, I feel different. Sarah Jane is part of two other sections of my life. And filed away with her is the pain of not being able to destroy my enemies when it would have taken no innocents.
"But humans decay. You wither and you die." Oh, gods, it's going to happen to her too. My blooming Rose. The withering isn't as bad, because she will always be beautiful to me. But she's going to die. And I'll die too. A larger part of me than when any of the others died.
"Imagine watching that happen to someone who you--" He breaks off suddenly. What do I feel about her? That she is fantastic, clever, the perfect companion. That she saved me in my very darkest moments. That I cannot bear to be without her. But is that love? It's been so, so very long; would I even recognize it for what it is? If it isn't love, it's strong enough. He had almost steeled his courage when--
"What-- Doctor?" She knows. She must know what I was about to say. I can hear it in her voice: the hurt that I can't say it. But what if I tell her and I'm wrong.What if it's not love at all that I feel? Or she doesn't feel the same for me? So he took the coward's path instead.
"You can spend the rest of your life with me." Unless I push her away so I don't have to watch her get killed. But it will be harder this time, to be without her. No, I will stay with her as long as she wants me.
"But I can't spend the rest of mine with you." I wish I could, Rose. But I must outlive everyone. They're all dead and only I remain. No, Rose, don't cry. You can be with me forever and forever as far as you're concerned.
"I have to live on. Alone." It's what I've always done. I should be used to it by now. But I never will be. I need people. I need an anchor. I want to be able to keep the memories and let that be enough. But it never is.
"That's the curse of the Time Lords." Her eyes widen in realization. There it is. The horror. I am cursed to carry the pain of the world. Is she worried that the curse will rub off on her. I won't let it. She will be happy.
And even the name Time Lord is so cursed that they are attacked almost immediately. But it is good, really, because it gives him something to think about.
There is no time to dwell on the past or the future.