Teenagers think they know it all. Sweeping generalisation aside - why? Well I think a lot of the reasons are hormonal of course, and with that raging torrent of hormones and the need to be treated more as a grown-up and less as a child, this is one of the attitudes they adopt. I think it could be because they can read, understand and write adult words. They can engage in nearly all the same activities an adult can (legality to the side for one moment) - sex, smoking, drinking, driving, drugs.. so they believe that as they can do all the things they see an adult doing, they think they must have cracked it, and believe they know it all.
But, as every non-self-delusional actual adult realises with hindsight - no, their teenage self merely observed the world of the adult - as per Stephen Fry above. They had yet to take personal responsibility in it, thereby fully understand it and thus also understand themselves and what they really want.
So, the trick is, how do you get that hindsight before you end up, years later, writing that letter to your 16 year old self that is full of regret and nostalgia?