For the first Generation X people who saw Star Wars in theatres as a kid in 1977 (like myself), our minds were simple - the bad guy was bad, the good guy was good, and "story" was basically about how the good guy would eventually beat the bad guy. In Star Wars, Darth Vader was the bad guy and wore black, pretty simple. At the end, his TIE fighter spiraled out of control and the theatres erupted with cheers - the bad guy was vanquished, victory won.
Enter The Empire Strikes Back and - hold the phone - the bad guy ain't so bad...is he? When he said those words "I am your father!", although we in the theatres gasped in unison with Luke "Noooo!", by the end of the movie - Luke outfitted with a new mechanical hand, Han Solo frozen in carbonite - it was clear there must needs be at least one more installment to complete the story even though we had indeed "searched our feelings, and knew it to be true."
