What are the main reasons athletes suffer cardiac arrest when they are young and in great shape?

As wonderful as medical science is, there can still be problems with the heart which go undiagnosed until something happens. Young athletes also have a horrible habit of taking things to enhance their performance, which can damage them internally. Athletes are also accustomed to dealing with a certain amount of pain while they are training or competing, and don't always speak up about it until something bad happens. They may also suspect a problem, but hope it will go away if it's ignored, just like most people. There are many reasons why this can happen, none of them wonderful though. Sometimes it is a combination of a minor undetected defect that is aggravated by things like drugs and the stress of the athletic training/competition. What would have never been a problem gets pushed into becoming one. And of course, sometimes there is just a natural weakness to begin with. It's always a shame, but unfortunately one of those things which do happen. Fortunately, not so often, but always a loss.