True fact: Best friends are always more interesting (and no, this wasn't an excuse to show a funny HP photo that didn't even relate to the post topic). It seems to be that the author wants their main characters/love interests to be consumed with each other, only thinking of rudies with the other person, and in doing so, they LOST THEIR PERSONALITIES. Or, the main character has some serious plot to get through (fighting Voldie, ahem) so the author doesn't have as much time to let their MC's personality grow. Instead, the funny one, the brainy one, the pretty one, the quirky one - they're all best friends.Picture this: A brooding, smouldering, posing hero - 17yrs old but writing poetry, playing kick-ass guitar, and saving the world becoause of some prophecy. He's mysterious, but his best friend - bit of a lad, likes throwing paper aeroplanes and smuggling in bacon sandwiches to his maths lesson, and borrows his sister's hair spray for a night out because next to smouldering, brooding hero he seems a bit inadequate - he is the realistic one. He's the type of boy that teen YA readers are more likely to meet, and the type of boy that would probably be a lot more fun as a date because he's not really tired after staying up to watch the heroine sleep...
The best thing about Best Friends - they're not perfect. And that means that they are instantly more interesting.