Why are sharks afraid of dolphins?

After reading Jera's entry, I've google for cases where man saved by dolphins. Every victim in the case was rescued by (wild) dolphins protecting ring. I wonder why the sharks went away. Are they afraid of dolphin? I found this :

"they are only afraid of dolphins when they are in with their pods... this is because the dolphins have their young with them and are protective and if a group of dolphins start charging a shark they hit their gills causing the shark to have loss of breath... some wat like humans when we are punched or hit in the chest.."

I also found another short explanation and now I know why there are so many cases like that. Beware of dolphins guys. Haha :)

Sharks aren't really afraid of dolphins; that's a common misconception that has led people to believe that when dolphins are present in an area, there will not be any sharks nearby. In fact, many species of shark and dolphin actually hunt and eat the same prey items, so the presence of dolphins often indicates the presence of sharks as well.

Dolphins and sharks are two ocean predators who have to compete for food and territory. They usually co-exist quite peacefully as part of a complex ocean eco-system, but occasionally they do attack and sometimes kill each other under various circumstances.