system
Given the following post, respond as if you're commenting on Reddit:
human
[Dr Who] Why does the doctor always seem sad when regenerating? What changes within his mind and being that makes him different?

We know it's the same doctor, the good doctor who still helps when he can, who does good and fights evil wherever he finds it etc just that he now has a different look to his face and body. But mostly, when he does change and his companions are near him, he tells it like all he is at the time is being erased and his memories just merged with someone new. Someone new who is still the doctor but just won't be him or her completely.
gpt
As Tenth showcases, while he can regenerate, he's still *dying* for this one. Regeneration was always treated as a big gamble, and while Ninth claims about getting two heads or no head are probably an exadderation, amount of times the regeneration went smoothly can be counted on fingers - there's always some weird shit happening from bouts of insanity, memory loss, and whatever that thing with the Fifth was. 

Regeneration is a time of ending, and the next one after you will never be the same.