1. Teenagers are very focused on self, their identities are growing and they are coming into their own.
2. Friends are more important than family, also family dynamics are huge plot lines. YA books often feature parental conflict and peer group situations.
3. Dating time! You have to allow for romantic feelings and sexual attraction.
4. Hormones are raging, kids this age are very emotional and focus a lot on relationships.
5. Unfortunately, kids 12+ are often prone to depression, loneliness and the feeling that their problems are unique. It is a good idea to allow them to experience these problems vicariously. It's why a lot of YA books are often thought of as "dark". Truth is, kids need to know they aren't the only ones who feel a certain way. They also need these types of books to show them how to overcome their problems. Like #2 mentions, parental help is often not there.
Take a look at several YA books, do you see these developmental stages represented as plots?
Inspiration taken from You Can Write Children's Books Workbook by Trace y E. Dils.
Image from http://www.squidoo.com/best-young-adult-books