Well, her boyfriend behaves in a paternalistic, condescending manner toward her. It's suggested that he's been supporting her, paying for her to go to school, but also that he doesn't have much faith in her. Maybe she doesn't have much faith in herself?
Until... she's forced to deal with things on the island.
So, I think 'sweetheart' might be what she has been, but won't be any more.
He also calls her "Sweetheart" at least twice. As she is the main character.... what better title for the movie.
