You're a teenager. The world is a lot more complicated than when you were a child. It's easy to find stuff to worry or wonder about: negotiating new freedoms with your parents, what your friends think of you, what's happening in school, whether the person you've got a crush on knows you're alive.
Even if all of this seems under control, you might find yourself spending much more time than you're used to worrying about your changing body. You might think your breasts are too large - or too small. Or that your penis is the wrong size. Or that you're too short or too tall. Or too hairy. Or not hairy enough. Or that your skin is a disaster zone.