I can be glad you've got people who care about you without knowing who you are, you know. I'd be glad if anybody's got people who care about them. It's a sad lot in life if you have to go it alone.

[ Whereas the Doctor's is a look of confusion. This isn't acting like he knows Mark. This is the Doctor being happy that Mark's got people who like him, people who he trusts enough to call his people. What's wrong with that? ]