There are several possible themes to discuss in "Romeo and Juliet."
It sounds as though you are looking for something that connects well with the definition of tragedy. In this case, one of the themes could be that unrestrained passion is fatal.
We see examples of this sort of thing throughout the play. Friar Laurence has several speeches that admonish against rash and violent actions. Tybalt as a complete character demonstrates the destruction of passionate emotions that are not balanced with dignity. From the very beginning of the play, unrestrained emotion drives characters to devastating violence.
A typical theme of "Romeo and Juliet" might be, simply, "love." But that is too simplistic.
The TRAGIC theme of the play is better stated as: "unrestrained passion can be fatal." Passion certainly was fatal for many of the characters in the play, and not just Romeo and Juliet.