The limbic system is the oldest part of your brain. Humans from the earliest points in history relied on their limbic systems in order to survive. One of the central features of your limbic system is your thalamus, which, among other things, instigates your fight or flight response.
In a world of primitive humans, the fight or flight response is the difference between survival and being mauled by a tiger. However, in our world of highly developed frontal cortexes, the limbic system can get a bit pesky.
Most teenagers never face death via tiger, but they do face everything else that could possibly happen in life, be it death, social expectations, dealing with people or more. Yet some of the fears most teenagers have are not only ridiculous but prevent us from doing things that are necessary for success.