So many YA novels feature protagonists who are rather cut off from others. They might have one best friend (two, max) with whom they share everything. Often they are alone, with their best friend having recently moved away. These characters speak to that common feeling of isolation and longing for connection that everyone feels.
And, let's face it, it's easier to write a smaller cast of characters.
That's something that strikes me about John Green's Paper Towns, although you also see it somewhat in An Abundance of Katherines: the characters have a whole group of friends. Quentin in Paper Towns has his two best buds, but there's a group beyond that, and Margo has her group. I like that. I like that Q is pretty well grounded and solid in his friends. (He even makes a joke about it--both his parents are psychologists, "which means I am very well adjusted.")
The tendency among writers is so often to put their students into troubled home lives. No parental support, they reason, means that no parents are making decisions for their teens--the teens are the driving force in the action. They have to be in YA. (Confession: my main character lives in an orphanage, so I am not above my own accusations, though in my defense the seed for my story was the location.) I went to a YA-writers conference in which nearly every group, when brainstorming the familial structure for a fictional character, decided that the mom was dead. Seriously?
It's nice to read books in which the main character has a stable family. Parents get along with each other and their kids, they just leave their kids enough space to have adventures on their own. Thanks, John Green.