What's more important to teens than their friends? All their drama stems from the friends (slash family). Being a teen is an interesting of life, and one full of pressures. Peer pressure is the common term used. It's not all bad peer pressure either. In some groups, you are encouraged to be a good person, rather than a bad ass. Then again, in other groups, you're encouraged to be a bad ass. Greasers vs Squares.
As you're reading a YA novel, you'll notice that the main protagonist isn't the only one with a voice and problems. Usually, there is a whole host of best friends, or friends becoming best friends, or best friends transitioning to new friends. But rarely is the main character alone. And, since the main character can't do everything, the best friends get to do the other stuff, and we can explore another way of life through them. Rampant by Diana Peterfruend has a whole host of girls with different personalities, and through each of them, we get to see how being a unicorn hunter has affected them. Twenty Boy Summer by Sarah Ockler shows how a whole family mourns the loss of a brother and son (and the effect it has on the main character's best friend).
Best friends can also provide one more crucial role (among many, including a sympathetic--or sometimes an unsympathetic--ear): comedic relief. I'm sure we're all familiar with Harry Potter. Take Ron, the best friend. He is awkward, quirky, compassionate, and completely a boy. He provides insights into the magical world Harry has never seen. I could go into all the complexities of their relationship and how real it is, but just read the books and you'll realize what a fantastic character he is. Without his brothers, Fred and George, in the scene, Ron provides the comedic relief everyone needs now and then. As the books progressively gets more serious, Ron's blunders get more hilarious. Simply with a word, look, act, he can relieve the building tension, give us a breather. And, what I love about him, is he is actually a very complex, deep character, though on the surface he may be awkward and weird all the time. Luna, Crookshanks, Hagrid, all the Weasleys, and Neville (among others) act in this capacity as well.
So, as you're writing your YA, don't forget the friends. They provide subplots that give another layer to your story (like an onion) and deepen it, add another complexity. Even Bella (though she forsook her friends) had the friend characters in Edward's family.
Based on best friends only, what are your favorite YA novels?