Buttercup is an inconstant bitch

Re-watching this, I remember now why I always disliked Robin Wright. She kept playing parts where she was a bitch inexplicably beloved of the hero of the film. In this one, she swears never again to doubt Wesley upon discovering him, and the first thing she does is forsake his love and send him off back to his ship. The first sign of danger, and she ditches him. But he keeps forgiving her... why? Because she's pretty? She doesn't seem to offer any other virtues.