Just listen to it.
Saw this episode of Sonny with a Chance the other day. For those who doesn't watch the show, Chad (Sterling Knight) is Sonny's (Demi Lovato) ex boyfriend so the song was meant to show Sonny's feelings towards him.
As Sonny's friend puts it: You're singing that song either because you want to get over him or want to show that you still love him. Either way, he's still in your system.
Wow, that's a good way of putting it.
Tell me what to do, oooh, about you ;)