One of the key themes of the play is that of the force of destiny and what happens if you try to act irrespective of the forces of destiny. This idea of fate runs throughout the play, from the first speech of the play when we are told that Romeo and Juliet are "star-crossed lovers." Both Romeo and Juliet see omens throughout the play, and when Romeo dreams of Juliet's death he shouts "Then I defy you stars!", which clearly shows he is implacably opposing fate. This defiance of fate results in the tragedy at the end of the play.
You might also want to think, apart from fate, of the various social barriers that there are that Romeo and Juliet transgress through their union. Clearly both of them are turning against the wishes of their families, especially Juliet, who would not have had a choice of who she would marry according to her parents. The law and the ways of conducting yourself are implacably opposed to blind passion and how we act when under the forces of love. Likewise their love does not fit in to the constraints of Christian religion, especially with their suicide at the end of the play.