Answer:
Well, there could be many reasons. Here are just a few.
Changing schools
Going on a first date
Having high expectations put upon them
Parents and teachers
Competing in sports
Participating in extracurricular activities
Having a job
Being over-scheduled
Not fitting in
Being ahead or even behind in physical development
Throwing a party
Family disfunction
School work/homework
Peer pressure
College/university applications
Anyone is free to add more if they can think/find more :}