I agree with #6. The themes of the story are universal - forbidden love, impetuous youth and the futility of feuds and war on any scale. However, I think it is the portrayal of these themes through Shakespeare's expert eye which gives us a play of great beauty and tragedy. The story of two doomed lovers was not a new one, just as many adaptations and variations exist today. The story was made synonymous with the characters of Romeo and Juliet by Shakespeare.
I agree with pohnpei in particular: the story appeals to the Romantic in many people, particularly today; whether it had the same impact during Shakespeare's era is another question. The most important reason that the play still appeals today is that it is wonderfully written.
Today, we remember the story because it's a story of youthful idealism fighting against the elders of the society who have forgotten what it is like to care about life. Young people often feel that their elders care only about things like jobs and money and getting by in life instead of truly living. I don't think that's probably what people saw in it in Shakespeare's time, but I think that's what resonates today.
I will confess that I don't know what the last part of this question is asking.
I do know that Romeo and Juliet was based on an Italian story that had become popular in England during Shakespeare's time. There were many feuding families in Italian politics, but to my knowledge the story is not taken from an actual historical conflict. It is a timeless story, I think, because it gets at some pretty fundamental themes- intolerance, fate, love, and many others.
I'm sorry... ^_^
I mean, What is the secret of the continuation of the story of Romeo and Juliet?