The Doctor watched helplessly as Astrid fell down . . . down . . . down . . . into the fiery inferno. Once more, he was powerless to stop yet another person dying for him. It seemed he only left death and destruction in his wake. No one was safe. That's why it was better for him to travel alone, to prevent himself from becoming attached to anyone and then putting that person in harm's way – all because of him.
Never again, the Doctor swore to himself. Yet he knew that his resolve would instantly weaken the next time he saw a pretty girl, and he'd convince himself to invite her along on his travels.
I've got to stop doing this to myself, the Doctor thought. I'm 903 years old. I should be used to this by now.
But who could ever get used to gaining so many companions, only to lose them in the end? Certainly not the Doctor – he may be an alien, but his feelings were very much human.
Later, though, as he stood conversing with Astrid in her ghostly form, he found himself at peace – specifically, with Martha walking away, and then, a short while later, watching Astrid fall to her death.
The Doctor bent down to kiss her, for what would be the last time. "Now you don't have to dream, you can travel forever," he whispered sadly. With his words, she dissolved into blue particles of stardust and flew away.
"Goodbye, Astrid Peth," said the Doctor under his breath. "I'll be seeing you."