A very long time ago, at the beginning of the temporal-wide conflict later referred to as The Great Time War, before the death of Davros and rise of the Nightmare Child, the Doctor was fatally wounded after crash-landing on the planet Karn. He had tried to save the last remaining occupant of a damaged spacecraft who declined his help once she realized her would-be rescuer was a time lord, a member a race which she believed had become twisted by their determination to win the war and were no better than the dreaded Daleks they fought against. The Doctor refused to leave her side, and so he perished with her as they fell from the sky all the way down to the surface of the planet. There the two were found by the Sisterhood of Karn, who dosed the Doctor with a healing elixir that revived him, but only for a few minutes. Just enough time for the acolytes to convince the Doctor to particiapate in the Time War, a service which they insisted was imperative to ensuring the continued existance of life in the universe.
Fearful of the many more live that could be lost should he not make a stand,
and realizing the redundancy of a Doctor in those desparate times,
he agreed to join the fight. To assist with his imenant regeneration, the sisters provided him with a selection of elixirs that could control the outcome of regeneration. Each was capable of inducing a particular characteristic ranging from weight, to age, to gender, to tempermant. They offered him an array of options so that he might have whatever it was he needed for the task that lay ahead. He asked them to make him a warrior,
and so they presented a potion latent with the qualities he required.
The Doctor sipped from the chalice, and seconds after, regeneration began.
He had been though the process before. But it was different this time. He didn't just feel as though he was changing physically and idiosyncratically as he had in the past. This was something more.
The transformation ran deep into his very soul. His memories and ideals were becoming foreign to him, as though they belonged to someone else. He felt himself slipping away as artron energy unravelled him to the core, and hurt in ways he had never experienced. Once it was over, the man he was before had dissolved, and in his place stood the new one. This man had no qualms about violence, no aversion to bloodshed, and no interest in retaining the title of Doctor. From that moment on, he was a warrior. This meant there could be no more jaunty excursions or lesuirely holidays through time and space, no more travelling with companions met along the way, no more upholding the values of pacifism as he had in his previous eight incarnations. His new life was one of duty, an immense responsibility to every innocent throughout the cosmos that ever existed. Life was no longer about living, it was about surviving.