One of the central ideas, or themes, in the play is the dichotomy between love and hatred. It's clear that it is the two families' hatred that keeps Romeo and Juliet's love separated. However, Shakespeare also shows us that there is actually a very thin line that can be drawn separating love from hatred, which he shows us through both lines and characters. In the opening scene, when Romeo first appears onstage and observes all of the damage the most recent Capulet and Montague brawl has caused, he reflects that a lot of the fighting has to do with hatred, but also with love, as we see in his line, "Here's much to do with hate, but more with love" (I.i.173). What Romeo is observing here is that it is truly the two families' passionate, stubborn love for their own ideas that is causing the hatred and the fighting. As we lean from the First Prologue, the fighting stems from some ancient conflict, meaning that both families have their own perspective on the way things should be, and it is their love for their own perspective that is truly causing the hatred. Even Juliet walks a very fine line between loving Romeo and hating him after he kills Tybalt. Hence, one main idea is that there really isn't a clear distinction between love and hatred and that love, especially selfish love can even lead to hatred.
Another central idea, or theme, found in the play is that violent, passionate, uncontrolled emotions have grave consequences, whether those emotions are hatred or love, which, again, Shakespeare portrays through both characters and lines. Benvolio is one of the play's more rational characters, and one of the only characters who emerges from the play unharmed. We see Benvolio's rationalism when he begs Romeo to listen to his advice and forget about Rosaline. Lords Capulet and Montague are guilty of acting upon violent, uncontrolled emotions, and they both lose their children to early deaths. Even Romeo and Juliet are guilty of violent, passionate, uncontrolled emotions, and just as Friar Laurence warns, their "violent delights have violent ends," meaning that their violent passions and lack of rationality are contributing factors leading to their early deaths (II.vi.9).