Regeneration was no easy task. Like dying, and then some. Every fiber of his being undone and then stitched back together again, but differently. Like amnesia in reverse. He can remember every act of every past life, but he doesn't know if he is right or left-handed, whether he likes apples or oranges. That's the thing about regeneration. It isn't just waking up the same man in a new body. It's waking up the same man plus a new man and a new body and having to figure out how those things fit together. Waking up, fresh from regeneration, he never knows exactly what sort of man he is. He is always the Doctor, of course. But what will his impulses be? His quirks? He doesn't know, inherently, anything about himself, until Rose.
Often, he regenerates mid-adventure. It is those adventures that tend to cause the regeneration, after all. It's a kind of rush, coming to and not knowing the plan, or even less of a plan than usual, because he doesn't know just what he is inclined to do. But it is also unnerving, not having any patterns of behavior to go on. There's a moment of blankness, a very deer in the headlights feeling, when the Doctor must be the Doctor and save the day without knowing, at the moment, just who the Doctor is. But that ninth regeneration, waking up in that tenth body, is different. Even in the midst of the regeneration, when he is being unstitched and restitched, as he is floating in the stifling, healing darkness, there is something nagging at his consciousness.
He knows, instinctively, that he wants to help her, save her, protect her. He always wants to help people, of course, to protect those who cannot protect themselves. And sometimes those that can. But he knows he needs to wake up, to reach the surface, because Rose, his Rose, is in danger. It is these thoughts of her, his beautiful, adventurous, brave companion, that bring him to the surface, maybe just a little too soon. He isn't fully charged, fully stitched back together, but Rose needs him. And so he rises to the occasion. Maybe it's because his last body died saving her, taking on all the energy from the vortex so that his Rose with her brassy, brilliant blonde hair, sassy cockney accent, and bright eyes that begged for adventure didn't burn up from the inside out.
She was so young when they began, still is really. And he is so old, has lived so many, many lives. And yet in all the 900 hundred years of his existence, he has never met anyone like Rose. In all the worlds in all the universes, she shone so brightly. He couldn't have stayed away if he tried. He was a husband and a father, with a wife and children, back on Gallifrey. But that was lifetimes ago. Even then, back when he'd had a family and a life and a people, he had never felt like Rose made him feel. Like he was more than just the one who ended the Time War, who ended two races. It wasn't even that she made him forget all the terrible things that had happened, that he had done, in the past. But she believed in him, and that belief was so strong and so pure that it made him want to rise above that past that he is so ashamed of. There is just something about Rose, something in her existence that calls to his.
The Doctor doesn't believe in forever. At least not in the way Rose does, not the same forever humans are taught to believe in. When she says he will stay with him forever, he knows that kind of forever isn't possible. But in that moment, with that girl, he wishes that kind of forever existed. That for the rest of time they could ride around together in his magic, bigger-on-the-inside box going on adventures and saving distant worlds. But of course that can't happen. Instead, he smiles when she says forever, cherishes that instant when the possibilities are endless, and satisfies himself with learning just what kind of man this new regeneration is with his Rose by his side.