I want to study Psychology because it is provides something for me that other subjects just don't. Since year 7 i've been stuck on Psychology and have never once though of it as a way to delve deeper into my own mind, but instead a way to understand other individual behaviour. I'm in sixth-form now (year 12) and I still love it so much, and I see a lot more teenagers who desire to learn Psychology for introspective reasons. It is one of the most unappreciated subjects due to its spoilt reputation and high drop-out rate of people who realise that they though Psych was going to be all "mind reading" etc.
Psychology focuses on human behaviour and the reasons behind this. Surely in our ever evolving society that is one of the most interesting topics to be discussed. Whether its memory, social psychology, cognitive, biological or whatever, the amount of newly found illnesses and syndromes popping up from everywhere needs more attention. Psychologies broad adaptability makes it one of the best subjects to me, as i feel that if you're not able to understand an individual in this society so many more problems can arise.
In terms of the reasons why teens pick it, it may be due to the slightly naive focus on themselves, but I also believe that those who are really serious about it know that behaviour of the individual, not just themselves or a specific person, is one of the greatest reasons to study the topic.