The bull shark is aggressive for two reasons: it's an apex predator who has no natural predators, and because of its physiology. Bull sharks are not only known to be one of the most dangerous sharks in the worldwide, but they're also considered apex predators because once bony fish, sea turtles and stingrays get too big and smart for the little guys to take 'em on, bull sharks are one of the only things left that can eat 'em. And they will. The other reason Bull sharks are aggressive is their physiology. They have a unique ability to stay fresh and saltwater, which gives them a competitive advantage over other marine predators. But this advantage comes at a price. In order to be able to pump oxygenated blood to their muscles in both fresh and saltwater, bull sharks have a very high metabolism. This high metabolism requires them to keep moving and HUNT for food constantly, which makes them aggressive.