Everyone likes Sandy. From what we see in the movie, Bunnymund is quite attached to the Guardian of Dreams, it's also said that Sandy should be almost as old if not older then Aster himself.
This relationship is based on respect, almost like the one Bunny has with MiM, but this one is more on a personal level, because Sandman and Bunnymund interact with each other, and the fact is that they communicate, even if Sandy doesn't speak with sound, but uses his dream-sand to do it.
It shows how close they were, the moment when Pitch's black sand takes over Sandy and makes him vanish, how along with North and Tooth he mourns his loss, but he knows that in his memory they must fight and win against the Boogyman.
And then when Sandman is back, thanks to children's belief, when Bunnymund
sees him he smiles that sincere smile commenting: "Mate, you are a
sight for sore eyes!"