Too much talk about their looks.
A lot of people have made comments that Sutter wasn't handsome enough, or that Shailene was too pretty to be a "quiet nerd" type who didn't get attention from boys.
Yes, physical attractiveness has importance in what kind of attention you get. But how you present yourself, and your personality, has WAY more importance. Sutter wasn't really handsome, but he was loud, outgoing, and apparently had a lot of charisma. I personally found him extremely off-putting, but for a large amount of high schoolers, his personality was appealing.
Aimee was quiet, self-deprecating, and didn't take much pride in her appearance. Her personality and interests would not have made her a popular girl in high school culture.
Maybe I understand this because I was an "Aimee" type, to a degree, in high school. I started wearing nicer clothes and styling my hair my senior year, and people considered me physically attractive. However, I was not popular. I was never asked out by guys in my high school. I was respected for being good at school and accomplished, but I never would have been popular in the prom queen/athlete way. Not by a long shot.