The Doctor burns cold. He moves through lives like a light, touching everything and nothing, there and gone.
"Not always," he says in stark counterpoint to, "Just this once, everyone lives."
"Do you know what's dangerous about you?" Rory says, and Amy smiles, says, "You really are an alien, aren't you?"
They come back, sometimes: Sarah Jane Smith and her unrelenting curiosity; Rose Tyler ripping through the walls of reality like a kid confronting drywall with a sledgehammer; Jack and his desperation, his fixed point in time (and what, the Doctor wonders in a distant part of himself, would happen if a fixed, immovable fact of the universe like Jack fell through one of the cracks, confronted that initial explosion).
(Sometimes they're just gone.)
"I am perfectly capable of admitting when I'm wrong," he tells Romana; to Susan, he says, "One day I shall come back. Yes, I shall come back."
("You don't always tell me the truth," Amy says.)
The universe is vast and beautiful; it always surprises him. Time is complicated. Time can be rewritten.
"Maybe when you're older," River says, and, "A very good man. The best I've ever known," and, "Oh, Doctor. Aren't we all?"
(Martha answers, "We could die any minute, but all the same—it's beautiful.")
"Don't you ever change?" Donna asks; says like a plea, "I was going to be with you. Forever."
(Donna Noble is the most important person in all of creation; Martha walks the Earth for a year to save the world; everything, all of time, hinges on Amelia Pond.)
Maybe—maybe his companions, they're the light, candles and fireworks, bright sparks and flashes across the night sky, distant pinpricks of beckoning life in a vast universe. Ace leaves, and Adric dies, and Amy has a white dress she plans to wear tomorrow, tomorrow, tomorrow. She takes the keys and reaches a frost-covered hand across the frozen Tardis floor to clutch Rory's fingers.
"I grew up," Amy says; the Doctor replies, "Don't worry. I'll soon fix that."
("The life out there, it dazzles," the Doctor explains. "It blinds you to the things that are important.")
The Doctor burns cold, but he doesn't burn alone.