I think that with the construction of any thesis statement, you would have to go back to the original task or writing assignment and see if there are certain elements that have to be integrated within it before you start composing your statement. Once you have done this, I think you should be able to proceed away. From a general standpoint, you could probably go in a variety of directions with the idea of young people in love. I think that you need to define what you consider to be "power" and "passion." In defining these terms in more specific contexts, textual support can be used in explaining their implications. An argument can be made that it is the youthful zeal of both of them that ends up bringing the families together at the end or that causes them to break family convention and seek to be with one another in the first place. An argument could also be made that the feelings of love that both youths share does not really solve anything, as Romeo still kills even though he is supposedly in love with Juliet. He might feel awful about it, but the fact remains, at that moment, familial bonds trumped young love. I think that assessing what is needed from the original assignment and then defining some of the terms in the thesis in clearer contexts will help in constructing a solid thesis statement.