Bad Wolf Bay
The Doctor wants to be able to say it, to tell her how he feels but he has to be strong. "I said Rose Tyler," he will not allow himself to cry.
She asks how that sentence ends and he can't ever tell her however much she begs. He has to remain aloof because Davros was right he is dangerous and one day Rose will get hurt or even worse she will change so much that Rose won't be Rose anymore. The other Doctor who pulled the lever and finished the time war once and for all by committing mass genocide, is free of the restraints that still bind the original son of Gallifrey. He bends down and whispers in Rose's ear those three words that have kept the Doctor fighting.
She holds the lapels of his suit jacket pulling him in for a kiss. The Doctor says nothing but his hearts are breaking and if he doesn't leave now the envy he is feeling will consume him. He knows leaving the other Doctor with Rose is the right thing to do as it ties up loose ends just as he put Jackie and Pete back together. He could never give Rose the life she so deserves more than anyone just like he can't allow Donna to stay as the DoctorDonna. At least Rose isn't crying this time, if she was that all important first step would be so very much harder.
He forces himself to turn his back on the young shop girl who killed the Emperor of the Daleks by pouring the time vortex into his head, he sent her away twice and she kept coming back. It was as she said that they could never be torn apart but the void closed and that was hard. This is even harder because it could have been him.
The familiar sound of the TARDIS pulls Rose away from the clone Doctor. She is so full with emotion that it's hard to stand, she never got to say good bye and she's sure he had his reasons but she feels robbed and more abandoned than ever. He probably had some ridiculously noble reason for leaving her like this and it was more than likely something along the lines of protecting her from another tearful farewell but how bloody well dare him? Who is he to say what pain Rose can endure?
She fought so hard to see him again just to be dropped off back safely behind the curtain between worlds out of harms reach, out of the way. He's out there amongst the stars living it up with Donna whilst she's left to walk the slow path just like Madame De Pompadour who died of waiting for him to come back.
The other Doctor holds her hand in silence and it doesn't make any sense but it's the Doctor. The same Doctor that she is so mad at who took her hand and showed her so many wonderful things like the Medusa Crusade and New New York. She gets to start again, he gave her that.
"I'm sorry," the Doctor says.
"So am I." She nods as they take their first step across the sand.