We all have ex-boyfriends, ex-girlfriends, ex-somethings. We loved them, laughed with them, left them (because, of course, they never left us. No sir). They shaped the way we view the world. They even contributed to the people we are today whether we admit it or not. :)
So, it makes perfect sense that exes show up in our writing, especially in the land of YA. At seventeen, all I wanted was a boyfriend. All the popular girls had boyfriends. All the popular guys had girlfriends. I needed in on that! It was the best of times...for a while. Then came the worst of times. Major heartbreak occured and left me and all of my friends (bless them) in a wallowing pit of ice cream-laden misery. I believe the concept of peanut butter pancakes with chocolate and marshmallow syrup came into existence over a break-up.
And the world is better because of it.
So, how do exes shape your story? Do they pine for your MC? Is there vicious hate going on? How about a best friend hooking up with an MC's ex? Oh, the possibilities! And all of them make for a dramatic, earth-shattering plot twist. Those only-too-real happenings keep me coming back to YA over and over, remembering my own experiences and wishing I had another chance to do it all again. :)