Sandy is one of the characters that Jack respects the most in my opinion, especially after he sees him how he handles combat, because one of the things he says to him, it's to remind him not ever to go on Sandy's bad side.
The fact is that, Jack likes Sandman's creations, the dreams, because he likes to dream, maybe not really sleeping, but daydreaming of how it would be if children actually believed in him.
It's interesting to notice, that if you pay close attention, the fact is that
they knew each other before the events of the movie, for example, how excited
Jack is at the beginning of the movie, when Sandy's sand is pouring in the city
towards the sleeping children, how angry he gets when the little man is "killed";
making the fight against Pitch a personal one, and the happiness he shows when
the dream guardian comes back.