If Charlie and Sage are meant to be, why can't Sage stop kissing Charlie's brother?

And why can't Charlie stop thinking about kissing the new boy at school?

Everyone at the Bexley School believes that Sage Morgan and Charlie Carmichael are meant to be. Even though Charlie seems to have a new girlfriend every month, and Sage has never had a real relationship, their friends and family all know it's just a matter of time until they realize that they are actually in love. 

When Luke Morrissey shows up on campus his presence immediately shakes things up. Charlie and Luke are drawn to each other the moment they meet, giving Sage the opportunity to spend time with Charlie's twin brother, Nick.

But Charlie is afraid of what others will think if he accepts that he has much more than a friendship with Luke. And Sage fears that if she lets things with Nick get too serious too quickly, they won't be able to last as a couple outside of high school and miss their chance at forever. The duo will need to rely on each other and their lifelong friendship to figure things out with the boys they love.