Great Whites are considered dangerous to humans due to their size and their curious nature. Humans put themselves in situations where a Great White may be present and the shark may not recognize them as anything but natural prey.
Bull Sharks hang around in shallow waters near the shore, and also where rivers meet the ocean, which is where food flows in. This quick and toothy shark has limited visibility due to the brackish water, and a low biting inhibition.
The Tiger Shark, because he swims in warms waters, is found more often than the Great White, but has been researched the least. When food supply is low, he can switch easily from one prey to another.
The Great White Shark can reach up to 20 ft. long and can weigh as much as 4,200 lbs.; the Bull Shark, 11.5 ft. and 700 lbs., and the Tiger Shark, 14 ft. and 2,000 lbs.
It is not the sharks who are dangerous; it is the circumstances people put themselves in that are potentially threatening.