Doctor Who: 10 Things Everyone Always Gets Wrong About The Doctor
3. The Doctor Often Experiences Post-Regenerative Trauma
From Castrovalva onwards, every regeneration has been followed by some degree of trauma.
The Fifth Doctor can barely keep it together for the majority of that serial, and later, the Sixth Doctor is so disorientated by the process that he throttles his companion. Then, in a post-regenerative funk, the Seventh Doctor believes the Rani to be Mel, and plays along with her scheme until he sees the error of his ways.
This tradition continued into the TV Movie, and then The Christmas Invasion. Thankfully, Steven Moffat appeared to dispense with this tiresome trope when he introduced Matt Smith, but when Smith regenerated into Capaldi, it was back to the out-of-control, unpredictable state of flux.
It's hard to see where this trope originated from, especially as the Second Doctor's companions are more put out by his regeneration than he is. His only concern is whether he can still call himself the Doctor, which he soon proves that he can.
It's likely the result of his forced regeneration into the Third Doctor that set the ball rolling, a process that has been seen to be fairly traumatic. Pertwee's amnesiac state in Spearhead from Space is likely what set the tone for the Doctor's subsequent regenerations - but thankfully, the Fourteenth Doctor knows exactly who he is.