He's playing Russian Roulette with Time.
Actually, she is. But he's letting her. You've got to let them make their own mistakes.
Not ones like this, something whispers. But he's too smart to listen to whispering voices in his head.
They go in and watch the car mow down Rose's father. The wire bends. When she fails to respond in time, they go in again. Three Roses within blocks of each other, two Doctors. The wire bends again, weakening.
She takes her father's hand. The wire snaps.
And this time there is no way to put dream and memory and logic back together, not until the Reapers have come and Pete Tyler lies down and lets the ghost car run him over and knock them all back into coherence again.
He won't play Russian Roulette with Time.
Three times he's thrown one switch or another, changed the present by turning gears in the past. Three times the other side has edited the timeline back and the Reapers have come and if they keep bending the wire back and forth it's going to snap, going to really snap forever. And this one is the size of everything.
He could set it back again, the way he did the first three times. Maybe, if he changed some details, it would work. Maybe they wouldn't find it the fourth time and reset events back to the way they happened / have happened / are now.
He slips along the probability-lines in his head and sees the narrow band of chances, the unstable path to the place where Gallifrey survives.
And on all sides of that narrow bright bridge, everybody dies.
If he tries to reset it again, and a single thing does not move exactly as he hopes, the Universe consumes itself. The whole thing. Maybe even all the Universes; he doesn't know; this is beyond the wide-flung limit of his understanding.
Or he could give Time the wherewithal to take its third path, the unspeakable one: the stable one, the one Time thinks is real. Edit the troublesome editors neatly out of the story. The Daleks die.
Gallifrey dies. But so many survive.
How many planets does he have the right to hang on a maybe, maybe, with lots of luck?
Not all of them. Tears blur his vision. Never all of them.
This time, holding Rose, cursing himself, he feels strangely light.
There was a chance, and they took it, and they still lost, didn't they? They still lost.
This time the oncoming collision wasn't his to choose or dodge, and he'd done everything in his power to stop it anyway, and the man who Time demanded should fall under the wheels chose of his own will to fall.
Rose is crying against him. "I'm sorry," she's saying. "I'm sorry. I'm such an idiot. I didn't mean to."
He wanted this, he knows, and he had to let Rose do the experiment. Nothing less wrong could have gone right.
Thank you, he wants to tell her; but he can't, because then she might realize.