A lot of YAs do end with a HEA, especially when it comes to stand alone romantic comedy, but when you're talking about series or other sub-genres outside of romance, there is the latitude for other kinds of satisfying endings.
One of my favorite teen TV shows, as you probably already know, was My So Called Life. The episodes didn't end with a happily ever after. They ended with more questions or some realization for Angela about her life. I loved that.
In my upcoming debut, there is a mixed ending -- happy, but not necessarily ever after, with new questions raised. That doesn't mean it isn't satisfying. You *have* to satisfy the reader. I can remember a few times reading a book only to get to the end and say, "What the heck?" because the ending a) doesn't make sense or b) cuts off .
As a writer, you have an unspoken contract with the reader. You're on a journey together that must come to a fitting ending. That satisfying ending is part of your building a relationship with your audience. And that relationship, no matter what artistic stance you take, is what writing is all about.
Happy Endings... or not,
Heather Davis is the author of
Never Cry Werewolf
coming from HarperCollins in 2008