It's Jack's idea: Christmas around the galaxy. The Doctor swallows his misgivings. So many of the celebrations involve fire. To locals, it's symbolic of light in darkness. To him, it's destruction and despair. He hides his pain with quips and the mask of the jaded traveler.
Rose and Jack join the bonfire dances on the beaches of Aquitano. The Doctor watches them; a motionless shadow.
Afterwards, the humans run to him, whooping with delight. Reflected in their young eyes, the firelight is transformed into something new. Though it doesn't yet stir hope in his hearts, it no longer brings despair.