Some narrative reasons for the regeneration into Tenteen (my new favorite name for him).
Thasmin reason:
Feeling so much love for Yaz, and relieved that Yaz didn’t have a gruesome end like some of the last few companions, and happy that Yaz will be able to continue with a normal life, but ultimately feeling so sad to let her go— all of this dredged up old feelings about the time Ten had with Rose, and how that ended similarly and — Oops! Old teeth!
Much more complicated Thoschei reason:
3 out of the last 4 times the Doctor has regenerated, the Master has been involved in some way.
End of Time: After a couple of years of some rocky back-and-forth, and the year that never was, and the Masterface!takeover and all that, The Tenth Doctor and the Simm!Master had the “get out of the way” moment of reconciliation against Rassilon. Then Ten died as a result of something else — not by the Master’s hand.
Eleven never knowingly met the Master which is a right shame really
The Doctor Falls: Twelve spent decades teaching Missy how to be good, which ended with Simm!Master and Missy teaming up, but then a semi-similar moment of reconciliation between Missy and Twelve right? Like she still betrayed him but she felt bad about it or something? I don’t remember super well. Anyway, shortly before Twelve died in the next episode, he was able to have a moment of hope for his old friend.
But Dhawan!Master didn’t give Thirteen any of that hope. He was just being angry and hateful. And this time, he DID directly cause the Doctor’s death. Now all those years with Missy seem to amount for nothing. The Doctor is once again grieving the loss of that friendship. How disappointing that they couldn’t team up to defeat a greater evil like they have before and — Oops! Old teeth!
Or maybe it’s just— “You know what? I don’t want to go, but that’s okay. I’ve matured since the last time I said that, and I’m handling this much better than I did back then aren’t I — Oops! Old teeth!”
Anyway I think the Doctor’s mind wandered to Ten at the last second, and now we have Old Teeth.
Thoughts?

















