Kristoff only ends up with Anna so he can exploit her sister's power in order to get ahead in his ice business.
--Of course, later on he realizes that nobody was getting anywhere with Elsa, so he switches his attention to Anna in order to indirectly have influence over Elsa.
--This motive is hinted at when he reasons that if he does not continue helping Anna, he will never get a new sled. And sure enough, he gets the latest model at the end of the movie.
--The audience knows that this is really the main point of the movie because the film begins not with Anna, Elsa, or Hans, but with Kristoff.
--And what does the song say during this scene?
this icy force both foul and fair has a frozen heart worth mining.
--But look at his face, Emily. Surely you can't deny that he loved Anna.
Well of course he looked disappointed. He knew that Anna was his last chance to get to Elsa's magical ice powers and he wouldn't be able to thaw her heart because he did not truly love her.
--He had the same look when he saw Elsa's ice palace. He was even close to tears then.
--Sure, he goes back for her at the end. But only because Sven convinced him to. And he's not the one to thaw the heart, so there's no proof that he loved her enough to do so. He probably just went back to make sure he got his new sled.