Why do so many young people risk their lives in sports like rock climbing?

There are many, many reasons why people do dangerous things. Interestingly, one of the main reasons young people do risky things is because from about age 13 or so until about age 25 (like when your car insurance goes down) people are undergoing a period of growth in the part of the brain that controls risk avoidance and judgement. What this means is their ability to correctly assess the danger of an act (like rock climbing) is not working properly. Most people get through this stage of human development with the help of their parents and older relatives. On top of this, young people have lots of energy, so they are always looking for something to do. Add that to the fact that new adventures are the most fun, so you have people who are less able to judge the risk of what they want to do, plus the desire to get out and do something plus the desire to do something new and different and you have your answer.