The Return of Doctor Mysterio - S10-E1
The Doctor: [Looking at the Superman comic] Take a look at that picture.
Young Grant: OK.
The Doctor: Now, this one. Take a good, long look. Takes a moment to see it.
Young Grant: See what?
The Doctor: Superman and Clark Kent are one and the same person.
Young Grant: Are you serious?
The Doctor: Yeah. Look, I drew specs on Superman.
Young Grant: Everyone knows they're the same person.
The Doctor: Uh. Well, Lois Lane doesn't. And she's a reporter.
The Doctor: Winning? Is that what you think it's about? I'm not trying to win. I'm not doing this because I want to beat someone, or because I hate someone, or because I want to blame someone. It's not because it's fun. God knows it's not because it's easy. It's not even because it works because it hardly ever does. I do what I do because it's right! Because it's decent! And above all, it's kind! It's just that...just kind. If I run away today, good people will die. If I stand and fight, some of them might live. Maybe not many, maybe not for long. Hey, you know, maybe there's no point in any of this, at all, but it's the best I could do. So I'm going to do it. And I will stand here, doing it, till it kills me. You're going to die, too! Some day. How will that be? Have you thought about it? What would you die for? Who I am is where I stand. Where I stand...is where I fall. Stand with me. These people are terrified. Maybe we can help, a little. Why not, just at the end, just be kind? (00:33:55)
Chosen answer: 1) When Stephen Moffat took over he ignored a lot of what had been developed before (there is not in-universe answer). 2) It would have killed Rose, so the Doctor absorbed the energy. His body regenerated before the energy could do a significant amount of damage that would prevent regeneration.