Question: 1. Why was Rose not allowed to touch her past self without creating a paradox and causing those creatures to appear and eat everyone, but Amy was allowed to touch her younger self without any repercussions? 2. Why was Rose able to have the time vortex in her head for a few minutes and it only knocked her unconscious whereas the Doctor had it inside him for about 30 seconds and it basically killed him and caused his regeneration?
Question: What were the circumstances behind the Doctor's regeneration, from his 8th incarnation to his 9th?
Chosen answer: The Eighth Doctor regenerated because he died in a crash on Karn, where the Sisterhood revived him to offer him the choice to either die, or regenerate to fight in the war. He choose to become the War Doctor, who regenerated after the events of "The Day of the Doctor" due to the stress of the Time War, or maybe because the war was over, and there was no need to be a warrior anymore.
Chosen answer: 1) When Stephen Moffat took over he ignored a lot of what had been developed before (there is not in-universe answer). 2) It would have killed Rose, so the Doctor absorbed the energy. His body regenerated before the energy could do a significant amount of damage that would prevent regeneration.