Because I find it utterly cruel to say that the Doctor, someone who will live centuries, maybe even millenia more, and is relatively young for his people, can't possibly ever love anyone else, or be loved by anyone esle ever again in his life. Also, that David Tennant should be the only person to play the Doctor, ever, and people hate Matt because of not being David. Ouch. Talk about cruelty.
This story was written because of these two things. So, what would happen if both things were true to the Doctor's character? Let's find out, shall we?
Forever, in the End
There were two things that were meant to last forever. His life and his love for Rose.
Only one was important to him, and that was Rose. Rose who he had died for once. Now, now he had promised himself to never die again, because she didn't want him to. So, somehow, he had to stay the same, even after he regenerates.
The energy of regeneration coursed through his body, and Rose was begging him not to change on her again. No change, no change. So, he didn't change. Instead he used the energy to heal his body and sent the rest to his spare hand.
The hand ended up growing into another, more human, version of him, and in the end Rose chose that one. So he was all alone. Cursed to live for the rest of Eternity alone, loveless and heartbroken.
The good thing about this was that to him, she would never die, just like his other companions he never met again, that lived and moved on while travelling with him. She would always be young and fresh and so, so wonderful, that his mind could never even imagine her any other way.
But he could still promise himself that he'd stay the same man for her. Never change himself. It was all he could do.
So, when he was stuck in that booth with the radiation running through him and he knew he would die, he panicked. He'd never had any luck choosing what he'd be like during regeneration. As it was, he had luck, because in the end, he was able to siphon off most of the regeneration energy, and was able to absorb the rest to heal his body.
He was still him. He knew what to do now, the way to cheat death, the way to stay him for the rest of eternity.
He'd do it for Rose, he'd do it for the love they had shared so briefly.
There would never be another for him. Ever. He'd never be able to love anyone ever again because of her, but that made him as happy as he could manage, which wasn't really happiness at all, more like longing. Because she was still Rose and that was all that mattered.
One love, with one life. He'd celebrate it all eternity by not changing himself.
After the first eight times he did this, he realised something very, very important. He was worthy of love, he was worthy of someone else, he was being an idiot. But it was so implanted in his subconscious now to not change, that when he went to regenerate fully, to let the process take its natural conclusion once again and become someone different, he failed.
He was the same. Same suits, same hair, same mannerisms and rudeness. Same everything.
He was stuck, forever, for all damn eternity, as this one man. A man who can't get anywhere good emotionally within himself, who was miserable, who wanted nothing more to die and move on like he used to be able to.
In the end, love turned to hate.
In the end, he cursed Rose's name.
In the end, he made sure she knew it.
He was able to send a message, much like he had the first goodbye, through a crack that had formed in the universe. He had told an old Rose, one with baggy skin and sparse grey hair and all alone since the other him had died young, what she had done to him. And she had been horrified.
In the end, she died with him hating her.
He was finally able to move on after that. He was once again able to regenerate, he was once again able to see reason. He was once again happy. This new incarnation, long due as it was, was young and flighty and just a tad bit crazy, but it was good.
May he never make the mistake of not changing for someone else ever again.
In the end, he had learnt his lesson.