10 MORE Unanswered Mysteries From Doctor Who
2. Why Did The Doctor And The Master Become Enemies?
This is one that has so many answers that it is hard to definitively pick the reason that the Doctor and the Master are enemies. Most histories of the two Time Lords agree that they were childhood friends, meeting on their very first day at the Time Lord Academy together. Having both looked into the Untempered Schism, the Doctor fled Gallifrey while the Master followed but only after going mad from what he saw.
From there, an explosion of possibility takes place. Certainly, they both riled against the Time Lords' non-interference policy, despite that rule being broken on many occasions. The Master's cruelty grew while the Doctor remained benevolent. At what moment they first came to blows, no one can say.
It is safe to assume a healthy jealously on the Master's part as this came to the fore during his discovery of the Doctor's true origin. This, in his mind, meant that the Doctor had always been something more than him, despite this not being her choice. Their years together, however, while featuring many deadly battles and devastating defeats on both sides, still contained a current of friendship between the two. While the Twelfth Doctor and Missy came the closest toward ending hostilities, the Thirteenth Doctor and the incarnation who had posed as Oh still were unable to kill each other.