I would say that one reason it is famous is definitely BECAUSE OF the sad ending. People love tragedy, and the idea of two people so young and so in love dying after one night together grabs attention. Another reason is that popularity is a self-perpetuating thing, and people love it because people love it.
Part of the fame of Romeo and Juliet comes from the fact that so many other love stories are based on this one. It is called the greatest love story of all time because it was one of the first stories of this nature. I think the sad ending helps the story and proves the point. If Juliet had run off to Mantua with Romeo, the story would be far more forgettable. Since both Romeo and Juliet go to such extremes to be together, we cannot forget them and their tragic loss.
Romeo and Juliet though written centuries ago still appeals to us with its relatable interpretation of tragic love. Love is something that will always exist in people's lives so the story is easy to relate to. Shakespeare didn't focus on the fairy tale ending of the story but rather the tragedy of love where enormous sacrifices were to be made although still no love was achieved in the end. The tragic ending leaves many in sympathy and evoked pathos which triggers more emotions from the readers than the usual, light and fairy tale-like stories.
I think the sad ending is actually a plus. There's something about a well-drawn tragedy that appeals to people.
I would attribute the lasting popularity of the play to Shakespeare's skill as a dramatist. I think the plot itself is too far-fetched to stand on its own. If just about anybody else had written it, Romeo and Juliet probably would have faded from memory long ago.
Romeo and Juliet remains a favorite love story because it appeals to many of our romantic sensibilities. The forbidden love. The pureness of first love. Love at first sight. The intensity of passion in youth.
We can relate to and enjoy the story of a love that seems meant to be, but is being forbidden by "the man", so to speak. We each know what it feels like to know/feel we are right, but be told no by those in authority. Because of that we read Romeo and Juliet and feel their pain and root for the young lovers.
Additionally I think its sad ending adds to rather than detracts from its appeal to readers. Most of us will never have the opportunity (good or bad) to die for someone or something that we love. Readers love the extremity of it. Our basic ideas of romance are upheld by the deaths. In many ways many people wish that they loved so strongly that they would LITERALLY die for it. It isn't happening in real life and most people would actually shy away from doing it, so Romeo and Juliet's exteme actions and passion, and sad consequences appeal to a part of us that we like to believe in, but don't really want to live out.
Romeo and Juliet is a favorite love story even today because of its tragic ending . It shows two lovers ignoring the hatred that goes between their two families . And at the end it shows how much they love each other that they were willing to kill themselves for they cannot live without each other . That is what makes Romeo and Juliet a favorite love story .