According to NOAA (National Oceanic and Atmospheric Administration) sharks do not normally attack humans, and when it does occur the reason is usually mistaken identity.

Sharks typically hunt sea lions, seals, and similar-bodied prey. So humans in the water on surf boards or kayaks can be mistaken as the same body shape as those normal food sources. Sharks can also investigate a potential food source by testing a bite. Unfortunately, due to the number of rows of teeth, a single shark bite can result in death by blood loss.

Beyond mistaken identity, shark attacks are generally considered to have four causes. The primary cause being provoked attacks, when humans in some way touch or disturb the shark. This includes fisherman releasing sharks caught in nets, divers taunting sharks, and other interactions.

The other three causes are unprovoked. The first, common of mistaken identity when investigating possible food, is an unaggressive shark that grabs a person, releases, and swims off. The others are aggressive, as when a deep sea shark comes up and grabs a diver unawares or when a shark comes up and head butts the person before taking a bite.

As of 2016, the International Shark Attack File through the Florida Museum of Natural History revealed the five year world average for shark attacks was 82 unprovoked attacks per year, with only 5-15 actual deaths per year for the whole world. Granted not all attacks will be reported, but this record is considered relatively accurate.

ISAF predicts that you are 132 times more likely to drown at the beach and 45 times more likely to be killed by a rip current than to be killed by a shark. That does not mean you should ignore caution toward sharks while swimming, especially in areas where sharks attacks are known to have occurred, but that there are specific behaviors you can do to reduce your risks. I recommend investigating sharks further at the ISAF website:
sharks

Many sharks eat large marine mammals, such as seals and sea lions. Seals and sea lions have dark colored coats that the shark recognizes as a prey item. Many shark species swim in the middle of the water column and attack prey from below. From the shark’s point of view, humans in the ocean that wear black wetsuits look very similar to a seal or sea lion. This is what often leads to shark attacks.

To avoid shark attacks, a company in Australia has made wetsuits that ‘break up’ the shark’s vision so that humans no longer look like prey items when the shark is looking up towards the surface – check them out!
no more shark attacks

Sharks are meat-eating animals and humans are made of meat. Also, sharks can confuse humans for their more usual prey and attack them by mistake.

Sharks eat meat, and we are meat. So if a shark saw one of us, it would think 'Food!'

Thanks for the great question.

Shark attacks are terrifying and dangerous, but fortunately they are very rare. About 70 attacks happen every year, but given the number of people in the ocean at any one point over a whole year, the chances of encountering a shark and getting attacked are very small.

There are nearly 500 species of shark, but only three are responsible for attacks on humans, the great white, tiger, and bull sharks. Despite being uncommon, shark attacks have serious consequences.

There are two different types of reasons why shark attacks happen: provoked or unprovoked. The first occurs when a person makes the first move towards the shark, for instance if someone makes the first move to touch a shark, or even accidentally bumps into one in the water. In these cases, the shark may attack because it is defending itself.

Other times, however, the shark starts the attack. These unprovoked attacks can be exploratory or predatory. The predatory shark attack, where the shark wants to consume someone for food, is the rarest type of attack, because people are actually a very poor source of food for sharks. More commonly, sharks bite because they are exploring their environment, and their mouth is the only way they can investigate their world. So when sharks attack, it is most often because they are curious about what’s in the water, not because they want to eat people. Unfortunately, a bite from a curious shark is still immensely dangerous.

Thanks for the great question,