The Doctor loves earth history, there’s so many interesting bits; kings and queens and pharaohs, great writers and composers, scientists and astronomers who sometimes get things mostly right… Earth’s still young by galactic standards so there’s a lot that humans haven’t figured out yet.
It’s not all good though. Great civilisations rise, only to fall to war, or natural disaster. He can’t change things that are meant to happen.
They’re in Pompeii, it’s volcano day, and everyone’s going to die.
Donna’s not having it.
They can’t save everybody; fixed point. But who would it hurt to save just a few?