It's actually sort of an accident--not that I write for teens at all, but that I write for teens only.
When I was a teenager, I was reading both adult books and teen books, back and forth. It was Paula Danziger's YA (or middle grade, by today's standards) Can You Sue Your Parents for Malpractice? that made me want to be a writer myself. When I was 18 and in college, I took a trip to Boston University to visit my BFF from high school, and that experience formed the basis of my first novel. At that very moment I happened to be reading one of my all-time favorite books, an urban fantasy YA romance called The Beginning Place by Ursula K. Le Guin, for the third time. I remember this specifically because I lost it on the airplane and had to look everywhere for another copy (this was before you could order anything off the internet). I finished writing that first novel when I was 20, and although I can see now that it was clearly a YA book, at the time I thought it was an adult literary masterpiece. All English majors want to be Hemingway, at least for a little while.
Well, my agent had the good sense to tell me that novel #2 was a YA rather than an adult book, but I did not have the good sense to take his advice. By the time I purposefully wrote a YA novel, #5, the market for YA had crashed, and nothing was beeing published for teenagers but those Goosebumps books and other horror stories. I got a "good rejection" from an editor for novel #7, saying that she would have bought that book if the YA market were better.
So I went back to writing adult books. But as I was finishing up novel #9, I saw that the YA market was finally making a comeback. I decided that novel #10 would be another YA. I knew of several authors who were publishing romantic stories for both adults and teens: Jax Abbott a.k.a. Alesia Holiday a.k.a. Alyssa Day, and Katie Maxwell a.k.a. Katie MacAlister. This was the career I wanted for myself. My agent sent novels #9 (an adult romantic comedy) and #10 (a teen romantic comedy, Major Crush) to editors simultaneously. We were both astonished that novel #10 sold, and novel #9 did not.
And that's where I still am, with one half of my career goal fulfilled. I have three YA novels out now, The Ex Games coming out on September 8, and two more YAs coming from MTV Books in 2010 and 2011, with no adult book in sight--not that I'm not writing them! I'm actually finishing up an adult urban fantasy/romantic comedy as we speak. But if I never got an adult book published, that would be okay with me too. I do love writing YA. And as long as I have an opportunity to write, I am grateful and happy.