Donna had never been particularly fond of history. At least not the dry, boring way they always taught it at school, with just names and places and dates. It wasn't real. It wasn't about things that mattered. She had learned about Pompeii at school - who hadn't? But it was just numbers, and a date, nothing to worry about.
But that was before she was there. Before she had touched the people, seen their faces, heard their stories. Twenty thousand is just a number. Twenty thousand people - with their own stories, sorrows, joys - that was different. And she had pushed him to see those individual faces when she was trying to convince him to change things. What good was it being a Timelord if he couldn't save people?
But then, in the volcano, came the realization that it was Pompeii or the world. That he didn't just let it happen, that he made it happen. And she had seen in his face the burden of a Timelord - to see what had to be, and sometimes to be the one to cause it. The burden of choice. The burden he had to carry alone, as the last of his kind. She put her hands over his, to share the burden and the responsibility. Knowing it meant death for the town, and certainly for them, in order to save the world.
But then they had a second chance, and she knew what had to be done. She tried to save the town, but they wouldn't listen. Why wouldn't they listen? And he was running, trying to escape. She begged him to just save someone, not the whole town, just someone. Trying to ease her own guilt, but also hoping it would help him. His own planet had burned - how often must he see that in his dreams? His face, full of loneliness and pain, was the one she saw in her mirror in unguarded moments, when she thought too much about the missing pieces of her own life. Caecilius had said they looked alike, Spartacus and Spartacus. He had thought it was because they were brother and sister. Donna knew it was because they were the same.