It seems like an obvious question, what with literally burning through your very being to form a new life filled with uncertainty, but why doesn’t The Doctor want to regenerate? We know that Peter Capaldi will leave the show after the Christmas special but it seems like the Twelfth Doctor will do anything to avoid another regeneration.
What are the reasons he extinguishes his regeneration energy in the snow during the series 10 finale, The Doctor Falls?
Expanding on both the process of regeneration and the Doctors reluctance to prepare for the moment, Steven Moffat explains in this behind the scenes video (via the Radio Times) that The Doctor just wants to be himself.

As Moffat explains:
“The turmoil of regeneration, while it extends your life, does sacrifice parts of you that you want to keep. It’s too radical a change. What [The Doctor] is saying is that actually, I can’t keep on being somebody else.”
Echoing our own trepidation over just who the next person might be, Moffat added:
“He’s frightened and horrified that yet again he has to do this ghastly thing where he won’t know what he looks like, won’t know how he sounds, what’ll he’ll end up with.”
Amen to that.
Doctor Who will return at Christmas.