They both had a fun madcap kind of quality to their performance, I felt she managed to pull off the crazy eyes even better than John Simm *g*

And I interpreted it as Clara wanting to tell him about Danny not being the one to come back, but the Doctor already seemed convinced that she was about to break it to him gently that Danny was back and she couldn't travel with him any more, so he was trying to gracefully step out of the picture and leave them to their future happiness by pretending that he had found his home. And then of course, once Clara heard that, she didn't want to put a dampner on his good mood, so she pretended to have found her happy ending as well. It was all so tragic