Why do sharks attack

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.