Because I'm sick and tired of the depression that welled within me when the 10th Doctor and Rose were separated. Let's just give them a logical, happy ending that they so deserve.
Disclaimer: BBC owns Doctor Who and his various companions.
He could feel it now, moving inside of him. Things were changing, his DNA was stretching and rearranging itself, coiling and uncoiling. His proteins were in a panic! A calming endorphin within his system released itself, allowing his mind to become fluid. full of nonsensical thoughts just before the change. It calmed him down, making sure that he didn't accidentally change something on his own without thinking and causing his body to mutate or malfunction. This was a delicate process, after all.
Once the change was complete and a new Doctor stood in the glory of his own anatomy, a thin stream of golden particles flew from his mouth. It swirled in the air, testing its new form, and looked back at the Doctor before zooming out of the TARDIS and into the vastness of time and space. The 11th Doctor watched it go with a sad smile. A part of him had disappeared forever, but he knew that it would find where it belonged soon enough. You see, Time Essence could in fact jump dimensions where the physical body of a Time Lord could not.
Time Essence release occurs every time one of the Lords regenerates his body. Part of his consciousness, usually a copy of the memories of the previous incarnation along with DNA codes to the physical form, are released in a molecular cloud resembling pollen. This means the that a human copy of the Time Lord's previous incarnation can be made anywhere in time and space, usually where or with who it was happiest.
The 9th Time Essence had found itself happily disappearing into the void, collapsing in on itself and dying. He had been happy that way, alone. But the 10th Time Essence had someone that he wanted to spend his life with. Someone who called to him, pulling his little cloud of molecular dust through time and space, jumping dimensions until it finally found where it belonged.
!~!
Rose Tyler stood on tiptoe in order to place a final decoration on the Christmas tree. Music from her radio played softly in the background. Her mother and father had gone to a party at the neighbor's house, and she was left to herself for a few hours. Deciding at last to really decorate for the season (which she and her parents had neglected for weeks) was a good way to waste some time, she did. Rose thought back on the Christmas before last, when the Sycorax attacked Earth. The Doctor had been there, in his new form, and she'd had to do a lot on her own or with Mickey, who was practically useless then. But he'd changed, and she'd changed too. The Doctor hadn't, even though his face was different. She'd been able to fall in love just as quickly with the new Doctor as she had with the old. Although, with the old, she hadn't been able to admit it to herself as easily. Maybe it was just his eyes...
The way they looked when...but her thoughts were interrupted when a thin line of gold mist filtered through the bottom of her front door and snaked across the floor until it was only three feet away. Out of habit, Rose reached for the TARDIS key she wore around her neck, backing away from the substance in case it attacked her. She was used to stray aliens recognizing her from her adventures and having to fend them off. She had adapted quite well to the life of a companion, and felt so lost without the Doctor there the past two years.
The gold mist swirled and danced, slowly forming from the feet up, a person. When the final product stood there, stark naked and slightly confused, Rose was aghast. They stared each other down. She looked at the mussed brown hair and the sad but seemingly endless love in his eyes. "Doctor!" Rose shouted, running into his arms. He was overwhelmed with the site of his Rose, the feeling of her safely in his arms once again. She pressed her head to his chest, trying to find his left-most heartbeat. When she failed she looked for the right. They were both so faint, they sounded distant; dying. He did not come all the way back just to DIE!
"Doctor..." she trailed off, looking up at him in confusion. Without speaking a word he took her head and placed it on the center of his chest and waited. Her eyes widened. Rose quickly pulled away and looked up. "But that means-"
"Yes. I have only one heart. I'm a human."
"But how?" she asked.
"How about I get dressed first, and then we discuss it over some tea. I'm really quite tired from jumping dimensions anyhow."
"But Time Lords can't jump dimensions unless it's an accident," Rose frowned, leading him to her father's room, where she quickly found him a pair of his favorite striped 'jimjams'.
"Technically, as you've already found out, I'm not a Time Lord anymore. Rose, I'm a human."
"Oh. How?" she asked. The Doctor explained how things worked and the mystery of the Essences. She nodded along with the story, at the end becoming rather upset. "Why didn't you TELL ME you could come back!? Why not? Did you like the fact that I would spend countless hours mourning your loss when I could have been rejoicing and awaiting your return to me!?" she shouted, pounding his chest with her fists in frustration. He marveled at the way his new human body registered empathy and pain. He grabbed her wrists and held her still, looking straight into her eyes and speaking evenly.
"If I'd have known when in your lifetime I would regenerate, I would have told you. I just didn't know if I'd make it back in time because of the way that time and space runs. I really wanted to tell you, Rose, I love you!"
"You what?"
"Rose Tyler, I love you."
"Doctor! I love you too, so much, and all these years waiting..." she tapered off as he leaned down and kissed her.
"I don't think The Doctor is a very human name."
"How about...David? David fits you," she smiled. The Doctor (now David) smiled at Rose and kissed her once more before bending to one knee.
"Miss Rose Tyler, Dame Rose of Powell, my dearest love in all the universes, will you marry me?"
"Yes!"
"Just like you said," David smiled, "You and me, forever."