Teen Brains Haven't Developed to Fully Understand Risk and Reward
Here's how stupid adults are: They all know that a human body goes through massive and often grotesque changes during the teenage years, but can't grasp that the brain is just another organ in the body, and as such, it's also undergoing horribly awkward changes. During the teenage years, every little calculation the brain makes is affected by a system that is in the process of trying to upgrade itself to adulthood.
So while grown-ups like to bemoan how modern teenagers are always running around doing stupid shit while high on bath salts, it's not totally their fault -- the teenage brain is hardwired to make it crave what adults write off as "idiocy." It's not that the teenage brain doesn't understand risk; when teenagers look at a dangerous situation -- say, riding a shopping cart down a steep hill flanked by cacti -- they know that there's a risk of winding up in the emergency room having barbs removed from their asses. But whereas an adult is likely to consider this an unacceptable risk, teens consistently put more weight on the possible positive outcomes, like impressing everyone and getting laid.
You can still bone with broken legs, right?
That's because, by design, the teenage brain is built in such a way that impressing other people is more or less the prime imperative, and when you combine this with the thrill of risk-taking, their brains are flooded with excitement. They literally get high off it. In general, this is a good thing for the continuation of the species since, you know, otherwise kids would never move out of their parents' basements and into the real world full of STDs and paperwork.
But before you say "This isn't a downside to being a teenager! This is what makes the teenage years awesome!" keep in mind that this same mechanism makes the brain predisposed toward mental illness and addiction. Advertisers know this, by the way -- that's why they pour millions into specifically advertising alcohol to teens, even though they can't legally buy it. They know that teenagers are insecure by nature and employ psychologists to help design their ads specifically to exploit those insecurities.