Sharks are not afraid of dolphins; dolphins only eat small fish. Sharks are eaten by orcas, so if they were going to be afraid of anything, it would be orcas.
Sharks are not particularly afraid of dolphins. Think of it as this:
A really strong shark comes to eat a pretty strong dolphin
Shark wins
A really strong shark comes to eat a pretty strong dolphin with another pretty strong dolphin
Dolphin wins
Yes, actually, they are. It's because dolphins always gang up on a shark when they see one, due to sharks' tendecies to eat baby dolphins, and the shark often from that attack.
