The volturi wants Rosalie. Emmett is going to stop them with his brothers, but he know that Rosalie would never let him go alone to the volturi. So he had to break her heart, the only option for him to know that she's save.
He may not have been Bella's choice but will he be Nessie's? - Nessie soon learns what it feels like to be forced into something to save her family. So is there really a such thing as Happily Ever After? - maybe, but not for everyone!