Thou shalt not kiss thy friend's boyfriend
Sarah wanted Ryan first, but Brianna, who is more extroverted and aggressive went for him. As the three of them started hanging out together, it became obvious that there was a stronger chemistry between Sarah and Ryan. Something needed to be done about it. And soon.
The Unwritten Rule is very repetitive despite it's short length. Brianna could be a charming girl and then turn into a terrible friend you wouldn't even wish on your worst enemy. This made it harder to root for the girl code. One wonders, how much deeper the book could have been, if Brianna had been the sweet girl next door.
On the plus side, the theme and situations (bowling dates, spending quality time with parents), made the story realistic. Elizabeth Scott also managed to give the ending a twist and provide the story with some great quotes on friendship and letting go.