https://www.vox.com/culture/24000340/doctor-who-the-giggle-david-tennant-russell-t-davies#:~:text=It%20is%20a%20betrayal%20of,own%20personal%20past%20especially%20so.

It is a betrayal of what makes regeneration such a durable and dramatically rich premise, which is that the Doctor can never fully regain his past self — and neither can the audience. Even if you are a Time Lord, the past is a foreign country, and your own personal past especially so.