Why do sharks attack

Because they are meat eaters and are looking for food, same reason we go to the grocery store!
Hunger.

If you mean why sharks attack humans, then this could be an interesting for you - sharks actually don't attack humans and in fact, they are afraid of us. The only thing that will make sharks attack humans is when they smell blood.

Sharks attack based on instinct. And, we do know that they attack humans, but many times they will only take a bite and leave once they figure out that it isn't their everyday meal. There is a case or two about this every year on the news.
Especially when surfing, sharks attack things that look like their usual prey like seals. So, its almost like they bite first and ask questions later. Blood could get them excited but many times people aren't really bleeding when they are attacked. They are just acting like an prey for the shark to snack on.

Sharks do not attack humans for the sole purpose of hunger. In fact, sharks do not know what the feeling of hunger is, and in fact, can go for many months without eating. This is not to say that sharks do not attack with the intention of seeking prey. Many attacks on divers and surfers especially can be attained to searching for food. To a shark, a surfer on a surfboard slightly resembles that of a seal or sea lion, or a diver in a black wetsuit can look like other prey.

Sharks also attack humans because they have been provoked or agitated by the person. Many spear-fishers have been attacked by reef sharks because when they spear fish, the blood from the fish and it's vibrations can sometimes result in a feeding frenzy by many sharks. Bright colours can also be counted for attacks. As many people have believed in the past, sharks do in fact can see colours, and do indeed have very good eyesight. Avoid wearing the colours of orange and yellow, as this can aggravate the shark, and possibly lead to attacks.

Sharks are in fact attracted by splashing and vibrations in the water, and it can sometimes be attributed to attacks. Most scientists have not been able to predict why and where sharks attacks.