Tonks was awfully energetic, and it actually made the Doctor feel quite energized himself. He felt like he was soaking up both her optimism and her jubilance - which was a good thing. He'd been surrounded by angst and discomfort far too long. It seemed to fester in the museum lately.
"Oh yeah?" The Doctor was intrigued. "That sounds like quite a noble calling," the Doctor said genuinely. "Seems like everyone here is fighting on the side of the good in their own way."