Why sports can be dangerous?

I guess doing any kind of sport can be dangerous, if it's done without proper control, I mean doing exercises can be helpful untill you feel that you are totally exhausted, and here your health can be in danger, especially your heart. So if you find a good personal coach and use such devices like Heart Rate Monitors, you may not worry. And you don't need professional versions that may cost a fortute, I think any like Beets Blu will do here.