The two most important themes of Romeo and Juliet concern the twin themes of love and fate. This is what makes the play so tragic. Romeo and Juliet fall in love with each other, but their love cannot come to fruition. They are head over heels for each other. As the work progresses, this love become more and more conspicuous. More importantly, the way in which Shakespeare describes this love makes it more powerful. Love is an uncontrollable force. Romeo and Juliet cannot control themselves. It is no wonder that that this love makes Romeo and Juliet go against all social norms, reason, and any other constraint that there might be. To make this love even stronger, at times Shakespeare describes this love in religious and magical terms.
With this said, there is another force, fate. Fate does not allow them to consummate their love. For example, the reason for the hatred between the two families is not given. It is just stated as a given. It is fate. The prologues even says that Romeo and Juliet are star-crossed, that is, they are ill-fated. As the work progresses, this is confirmed as the only way the two can be together is through death.