Because it's fun. It's really as simple as that.
I had 3 "adult" books published before I wrote my first teen book. I never really intended to write a teen book, it just sort of happened when my agent said, "Hey, got any ideas for a teen book?" But then I had so much fun writing it, so much more fun than I had writing my adult books, I wrote more. And now I think it's all I want to write.
What it comes down to for me is this:
1. When you're in your teens life is all about possibilities, there are so many unknowns ahead, so many choices to make, each with a different potential outcome.
2. Life is so less routine when you're in high school, you switch classes every hour for god's sake, show me a career where you're forced to shift gears dramatically every single hour! Not likely. So you get exposed to so much and have so much more to react to.
3. I had a really good time in high school.
4. Teens have so many different relationships (friends, teachers, parents, siblings, boyfriends, coworkers) yet they're all about to change at any time because that's the nature of your life - it's a constant state of moving forward into the unknown.
I tried so hard to find a picture of me when I was a teenager, like Danielle included in her post. The thing is, my parents have all the pictures taken of me back then. But I found this in a box with my old report cards. It's an ad for a bridal store. One day I walked into the bridal store to look for a prom gown, and they asked if I'd like to model in a fashion show. Then they decided to shoot an ad. I was all of 17 but every 6 weeks I got to wear amazing dresses and prance down a catwalk at these shows, it was like playing dress up. I loved it.
When's the last time I went from shopping in a store to wearing amazing gowns in front of hundreds of on-lookers? High school. That stuff just doesn't happen when you're a grown up.
And that's why I write teen books.