The Doctor was curious by DDR , he hadn't seen a game like it, he wanted to play it agaisnt Jack after and Arwen. He also was interested to see what would happen with her facing the guy who hadn't be beaten yet. He was still curious how the game worked as well.
"Ohh he must be good then," The Doctor said in response to Jack's comment. He knew Jack was interested to. "I'm surprised Arwen is going to try."
He knew she must have a lot of confidence to do so.