Why? It makes total sense that he'd be moping. More now than ever. They were literally his family.
Because Tennant spent two seasons moping over Rose (actually, longer than she was companion). Not only was it REALLY annoying, it was also unfair to both Donna and Martha (REALLY unfair to Martha - she could have been a GREAT companion if not for her crush and the moping over Rose).
I can take the moping for the first quarter of the episode, but then it's time to move on and move past Amy. For good. No more of this "I'm so sad Amy isn't here" or "Amy would know what to say/do".
Speaking of... I loved Amy (and Rory) as a companion, but they need to stay gone. They can never come back - not only would it ruin/cheapen their goodbye, but it would cheapen the very characters themselves.
Basically, everytime I even hear the name "Rose" I want to puke now. Her ending in Doomsday should have been the last we ever see of her and, the moping in Runaway Bride should have been the last we heard of her - it's one of the reasons I'm glad Tennant is gone, he never moved past Rose.
With Matt Smith, it seems like we were FINALLY past all of that - and we'll see now how he moves to a new companion. Obviously there's going to be some of that, but if they can contain it to the Christmas special and, basically, make a fresh start with the new companion, then I will be REALLY happy.