Doctor: You can spend the rest of your life with me. But I can't spend the rest of mine with you. I have to live on. Alone. That's the curse of the Time Lords.
Sarah-Jane: I had no problem with space stuff. I saw things you wouldn't believe.
Rose: Try me.
Sarah-Jane: Mummies.
Rose: I've met ghosts.
Sarah-Jane: Robots. Lots of robots.
Rose: Slitheen. In Downing Street.
Sarah-Jane: Daleks!
Rose: Met the Emperor.
Sarah-Jane: Anti-Matter monsters.
Rose: Gas masked zombies.
Sarah-Jane: Real living dinosaurs!
Rose: Real living werewolf!
Sarah-Jane: The Loch Ness Monster!
Rose: Seriously?
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.