Snyder, Susan. "Romeo and Juliet: Comedy into Tragedy."
Essays in Criticism 20 (1970): 391-402.

Thesis: In the author's words:

Romeo and Juliet is different from Shakespeare's other tragedies in that it becomes, rather than is, tragic. Other tragedies have reversals, but in Romeo and Juliet the reversal is so radical as to constitute a change of genre: the action and the characters begin in familiar comic patterns, and are then transformed -- or discarded -- to compose the pattern of tragedy.    (391)

Persuasive Points: Also in the author's words:

The feud itself seems more a matter of mechanical reflex than of deeply felt hatred. . . . The "parents' rage" that sounds so ominous in the Prologue becomes in representation an irascible humour: two old men claw at one another only to be dragged back by their wives and scolded by their Prince.    (393)
Mercutio has been almost the incarnation of comic atmosphere. He is the best of game-players, endlessly inventive, full of quick moves and counter-moves. Speech for him is a constant play on multiple possibilities: puns abound because two or three meanings are more fun than one, and Queen Mab brings dreams not only to lovers like Romeo but to courtiers, lawyers, parsons, soldiers, maids. These have nothing to do with the case at hand -- Romeo's premonition -- but Mercutio is not bound by events. They are merely points of departure for his expansive wit. In Mercutio's sudden, violent end, Shakespeare makes the birth of a tragedy coincide exactly with the symbolic death of comedy.    (395)
It is not only the shift from comedy to tragedy that sets Romeo and Juliet apart from the other Shakespeare tragedies. Critics have often noted, sometimes disapprovingly, that external fate rather than character is the principal determiner of the tragic outcome. For Shakespeare, tragedy is usually a matter of both character and circumstance, a fatal interaction of man and moment. But in this play, although the central characters have their weaknesses, their destruction does not really stem from these weaknesses. One may agreee with Friar Laurence that Romeo is rash, but it is not his rashness that propels him into the tragic chain of events but an opposite quality. In the crucial duel between Mercutio and Tybalt, Romeo tries to make peace. Ironically, this very intervention contributes to Mercutio's death.    (401).

Bottom Line: Good stuff.