Kristoff only ends up with Anna so he can exploit her sister’s power in order to get ahead in his ice business.
–This was clearly his idea all along, beginning from when he was a child. He noticed Elsa’s power when they were children and began thinking about the possibilities.
–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.