Jamie weighs whether or not it would sound too much bragging if he said 'to save the universe!'

The answer is, yeah, probably, and anyway she'd probably just raise her eyebrows at him again.

"Well, I can't go home," he says instead, with a shrug.

And then, since this is a multiple-sentence explanation and his elbows are beginning to et sore, swings himself around and into the chair. "So I had to wander about for a while and -- well, it's all a bit complicated, but it turns out the worlds sort of need someone to keep at it, and that someone's me. Universes? I'm not one for the technical terms."

. . . well, it's not that he minds a bit of bragging.