Answer:
Shark are dangerous because they are probably the most efficient predators on the Earth.
Besides, the human capabilities on water are very limited compared to the abilities of sharks in their natural habitat.
Some shark species are aggressive, and the smallest provocation will lead to a violent reaction or attack.
Links to other pages in this site
Where Do Sharks Live?
Why Does A Shark Grow New Teeth?
Why Do People Kill Sharks?
How Fast Does A Mako Shark Grow?
How High Can A Shark Jump Out Of The Water?