In Shakespeare's Romeo and Juliet, how are Romeo and Juliet responsible for their own tragedy?

It is difficult to really argue that Romeo and Juliet were responsible for their own tragedy, because the Prologue makes it clear that they were "star-cross'd," meaning that they were fated to meet, fall in love, and ultimately perish. Indeed, their deaths reconcile their feuding families, suggesting that some higher purpose was served by their tragedy. That said, maybe the strongest argument for Romeo and Juliet's culpability might be made by looking at the concept of familial loyalty. Both of them were bound by ties of blood to serve the interests of their families, and each violated this responsibility by pursuing their powerful desire for the other. Juliet, in particular would have been expected to obey her father's wishes in marrying Paris (though by the time the wedding was actually arranged she had married Romeo) and by refusing to do so would have been rejecting the natural, divinely-ordained order of things. Once they made this decision, some might have argued, their deaths were inevitable. In short, both Romeo and Juliet were aware of the boundaries and the rules of the world they inhabited, and they knew that they were violating these norms. By doing so, they risked their own lives, and could thus be said to be responsible, if not necessarily worthy of blame, for their own deaths.