How does the theme of Romeo and Juliet relate to the tragedy of the play?

The main idea of Romeo and Juliet is tragedy, star-crossed lovers. Without the love between Romeo and Juliet, there would be no story line for the play (or a title). Everything else about the play leads back to the theme of un-allowed but un-dying love.

Each character in the story, and even how they interact, adds more depth and meaning to the fact that there is no possible way for Romeo and Juliet to be together; yet, despite it all, no matter how others try to stand in their way, they refuse to give up the love held so dearly to them. The family feud, as irrational and childish as it may be, is a further implication that everything in the play revolves around the many, many reasons to keep Romeo and Juliet apart.

All in all, the "star-crossed lovers" idea is the theme of the whole play, with any and all other facets pointing back to the fact that, no matter what, love endures all. In this way, Romeo and Juliet becomes the most classic love story of them all.