Very excitedly, the Doctor and Rose go check out the futility on the monitors, and his grin falls like it was meant to, and she asks a meaningless question as his head drops to his knees. The Doctor gives her a lie in response, because he loves her. But Rose know something is up: "Is that bad? How bad is it?" And oh, the perky spark in his eyes, bright like a fever, as he jumps up, to anyone's eyes overjoyed. "Rose Tyler, you're a genius!" He plants a kiss on her head, but it's a shadow, an echo, a lie: "We can do it! If I use the TARDIS to cross my old timeline.... Yes!"
The Doctor fires himself toward the TARDIS. Rose follows close behind. If he just keeps talking he won't have to say anything; he won't betray himself. Remember Pete Tyler's apartment? How she just kept talking? "Hold that down, and keep position!" the Doctor shouts, excited, pointing at a level, ignoring Rose's questions. She holds it down, unknowingly canceling the forcefield, giving the TARDIS a way out. The Doctor: "If I'm very clever -- and I'm more than clever, I'm brilliant -- I might just save the world. Or rip it apart." She says that she'd go for the first one, cutely, and he agrees: "Now, I've just got to go and power up the Gamestation. Hold on!" He runs out again, so happy and full of energy that you know the Daleks are screwed. And the TARDIS door shuts behind him, and his face goes cold, and he stops running, and turns around.
Inside the TARDIS, Rose waits for him, equally excited, loving this part. The Doctor raises his screwdriver and points it at the TARDIS. And the engines start to run. "Doctor, what're you doing?" Rose shouts from inside. "Can I take my hand off? It's moving!" And outside, the Doctor keeps the screwdriver steady. Rose panics and tosses herself at the doors. He's always leaving, and it always hurts, no matter how many times it happens. Ask Jack in an hour. Rose: "Doctor, let me out! Doctor, what have you done?" And the TARDIS fades away, and the Doctor's alone. It's been bad before, when they've been separated, but at least when Rose died he could cry about it. This time it was his idea.