Death
In this article from Radio Times by Ellie Harrison, Peter Capaldi gives his thoughts on why death means so much to Doctor Who. His view is one which does seem to hit the nail on the head.
Of all the television shows to be created and aired, Doctor Who has remained on the air the longest. Now, there is a reason for that and Capaldi believes one reason is death. Doctor Who is the only show where the title character dies and then returns with a different face. When a new actor takes the role, the Doctor takes on a different personality.
Capaldi believes that the changing of the title character is a big reason why Doctor Who still airs. The idea of the Doctor dying is an ingenious way to bring a new face and personality to the program. The change is what Capaldi believes keeps Doctor Who on the air.
I see the point he is making here. People see death every day. We all lose friends and love ones. After we lose them, we must carry on. We know our loved ones don’t come back, but some wish they could to spend more time with them. Everybody can relate to this. We do have loved ones who we seem not to spend as much time with as they would like. There may be things we want and need to say to them, but we never get to say. So, yeah, we all can relate to this.
The best thing the producers could do in 1966 was to regenerate the Doctor. I am sure the popularity of the program is why they did this. Figuring out a way to have the show remain on the air was imperative. The death of the First Doctor was the beginning of an idea which keeps Doctor Who fresh and new.
Doctor Who would not still be on the air had the producers not regenerated Hartnell into Troughton. So, we have them to thank for the longevity of the program. We also have them to thank for the newness we get with every new actor playing the Doctor.