I think that the purpose of Casablanca was purely to entertain the people watching it. Sure you could read into the former lover truly letting go of a person. The fact that war is an annoying and terrible thing that ruins a lot of peoples live and throws off the semblance of a “normal life.” You could also argue that letting go of your past is a key to moving onward in your life, since it happens to the main character twice. But at the end of the day, despite the good amount of emotion, the complete story, the great score and the fantastic acting, I really do think the main purpose of Casablanca is to entertain. For this film, it really is up to you to find what reaches out to you when you watch it, and I was thoroughly engrossed.
