Why is the Shakespeare's Romeo and Juliet usually described as a tragedy of circumstance?
Due to Aristotle's argument laid out in his Poetics, a tragedy has come to be understood as a storyline in which the hero suffers, albeit unjustly, due to some tragic flaw. The flaw is usually a character flaw. However, in a tragedy of circumstance the tragic hero is born into his/her situation, or has it thrust upon him/her in some way, rather than it being the result of some fatal flaw. There are several circumstances that Romeo and Juliet are born into that lead to their tragic deaths.
One circumstance is of course the fact that they are both born into families that stand on opposing sides of an "ancient" family feud (Prologue, 3). The hatred that the two families share eventually leads to the deaths of their children. The hatred also feeds the character flaws of certain characters, such as Tybalt's hotheaded temper, which also leads to the deaths of both Romeo and Juliet, as well as Mercutio and Tybalt himself.
A second circumstance that leads to their deaths is the fact that they are both very young. Juliet is only 12, while Romeo is probably in his late teens or early 20s. It is their youth that explains their impetuousness, especially their impetuous decision to marry so suddenly, in secret, and without parental consent. It is also their youth that explains their unabating passionate feelings, especially Romeo's heartbreak over Rosaline. It is also their youth that explains some of their fatal decisions. For example, when Romeo saw in the tomb that Juliet's face still had color and that, "Beauty's ensign yet / is crimson in thy lips and cheeks," had he been older and wiser he might have been able to make the logical connection that she was not truly dead, for dead individuals have no blood running through their veins and thus no color. Had he been able to make that deduction he would have been able to spare his life and Juliet's own and make Juliet's plan work, despite the fact that news of the plan had not reached him in time. Hence, the circumstance of their youth also leads to their tragic end.