Pilot fishes can't hunt or defend themselves from predators, so they rely on Sharks for survival (they clean after them in exchange for protection).
Max hates David because he's a very innocuous shark who often gets beaten up in his place rather than fending off enemies;
He is however forced to stick with him since no other merfolk is willing to put up with his antics and rudeness. Max is an angry, jaded eleven-year-old who is extremely pessimistic and seems to despise a wide variety of things.