What are the main reasons why sharks attack people?

1. Most shark attacks on humans are often be interpreted as the shark mistaken the person as their food source (seal, manatee, etc.). A surfer, for example, ona surfboard with legs outstretched looks like a manatee outline from beneath the water.

2. With overfishing in some areas the shark's food sources have moved or been depleted making the sharks swim closer to populated beaches looking for food. This makes shark attacks on people more likely.