More than any character per se, the prevailing characteristic of all the major personages of Shakespeare's Romeo and Juliet, impetuosity, contributes more than anyone or anything else to the tragic elements in the play. It is this element, "these violent delights" that precipitates the violent ends.
Ironically, it is Friar Laurence who is as impulsive, if not more than any other character, considering his station in life and age that should give him more grounding. While he counsels both Romeo and Juliet against acting in an impulsive nature, he quickly marries the couple, hands Juliet a sleeping potion only after their brief conversation, and then rushes from the tomb knowing that Juliet will soon awaken. He, more than any single character is directly responsible for contributing to Juliet's despairing act of suicide, which in turn induces Romeo to do the same.
In Shakespeare's Romeo and Juliet, there are both characters and ideas that contribute to the story's tragedy.
Tybalt is much to blame for Rome and Juliet's death. He is much too eager to pick a fight: he wants to kill Romeo (for an imagined slight), but starts out taunting Mercutio. As they fight, Romeo enters and while he tries to stop the fight, Tybalt reaches around Romeo and takes a cheap shot at Mercutio—killing him. It is at this point that Romeo goes berserk and kills Tybalt. For this, Romeo is banished from Verona, and the distance between the couple causes a lack of communication whereby Romeo kills himself, thinking Juliet dead—and when Juliet wakes and finds Romeo, she also kills herself.
Capulet is another character that contributes to the death of these sweethearts. In the beginning of the play, he tells Paris to wait two years before trying to marry Juliet. And even then, he declares, she must agree. After Tybalt's death, Capulet changes his mind, agrees on a date (very soon) with Paris, and tells Juliet that if she does not comply, he will drag her to the church or throw her into the streets. Juliet, of course, is already married, and so is driven to extremes, through the Friar's plan—for without an option, she promises to kill herself.
Some people blame the Friar for marrying the pair, but he hopes to resolve the feud between the families with this wedding. His plan to have Juliet meet Romeo in Mantua, as well as to give Juliet a drug that makes her appear dead, is not a fault in my view because Romeo and Juliet both come to him for help, and each threatens suicide if he/she cannot find a way around the seemingly impossible obstacles that surrounds each.
The main idea that causes the death of these two young people is the concept of "fate." Before the play's action begins, the Prologue (which serves as a "chorus") explains the plot and ending of the story in that the two will perish because they are star-cross'd lovers," and it is their fate to die. No one can overcome fate, or so the Elizabethans believed.
Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the loins of these two foes
A pair of star-cross'd lovers take their life;
Whose misadventur'd piteous overthrows,
Doth with their death bury their parents' strife.
(Opening Prologue, 1-8
The other very important idea, or element, that causes so much trouble is the feud. It immediately pits the lovers' against each other, not because of their own beliefs, but for the resistance they will face from their parents before they can marry—and rather than wait, they secretly marry. For how could they surmount this kind of obstacle?
Romeo observes, "Here's much to do with hate, but more with love." (I.i.175)
And Juliet notes:
My only love sprung from my only hate! (I. v.138)
However, even in light of this, the two young people try. The feud has been going on for so long, that no one can remember how it started. The argument is feeble enough that with the death of Romeo and Juliet, the fighting immediately ends and too late the families make peace. If not for the feud, the play would have been very different.
In support of #4, I will point to a major theme of Renaissance culture: reason vs. passion. It is astonishing how often this idea is stressed not only in the literature of the period but also in the other writings of the time. Very few people in the play behave reasonably; they allow their passions (erotic, violent, or otherwise) to overwhelm their better judgment and to control their behavior.
Of course, almost all people succumb to this temptation continually in their lives, and so the play is not so much about Romeo and Juliet as it is about a common flaw in human nature. W. H. Auden once wrote that the reaction we have after witnessing a tragedy written in classical Greece is this: "What a pity it had to be this way." He wrote that the proper response to a Christian tragedy is this: "What a pity it had to be this way, when it could have been otherwise." In other words, classical tragedies tend to emphasize the power of fate; Christian tragedies tend to emphasize the failure to exercise free will properly.
An old but still valuable book on reason vs. passion in Shakepeare's plays is by Lily B. Campbell: Shakespeare's Tragic Heroes: Slaves of Passion.
One of the ideas that contribute to the tragedy of Romeo and Juliet is the idea of the just street duel. It is deaths from just such duels that set up the background to the tragedy specific to Romeo and Juliet. Another idea that contributes is the idea of the rightness, indeed the necessity, of vengeful family feuds, feuds that encompass extended families. These are both ideas the Prince was trying subdue and combat.
I thought I would reach beyond the typical love story of Romeo and Juliet this time and suggest an idea (as opposed to a character) that contributed to this tragedy: the pointlessness of family feuding.
Of course, Shakespearean tragedies always end on a hopeful note, and Romeo and Juliet is no exception (seeing that the big feud is ending by the last scene of the play), but one cannot deny that it is the feuding Capulet and Montague families that contribute much to the deaths of these two lovers. Simply said, if the two families were not feuding, Romeo and Juliet would most likely have been encouraged to pursue their relationship with gusto.
Further, feuding families have been the focus of other tragedy in literature as well. Consider the Grangerfords and the Shepherdsons within The Adventures of Huckleberry Finn. Huck, himself, realizes how very pointless the feud is and the dramatic irony, of course, is that we know Huck is wise indeed to realize this pointlessness while Huck considers his thoughts to be a flaw in character.
Finally, I thought it would be fun to give a modern day example. In the South today (where I live), feuding families remain very much at the forefront of life in the country. There are two families on the mountain where I live that have vowed to hate each other until they die. Just recently, our community was desperately trying to pave our road. One family (who lives at the top) encouraged the effort, contributing more than their fair share. The other family (who lives at the bottom) purposefully enlisted other neighbors to block the effort by not contributing, therefore nixing the project.
Yes, yes, … family feuding is still very much alive!
I think, to pick up on ideas expressed in both of the posts above, that the true tragedy of this excellent play comes from the way that Romeo and Juliet both desperately try to struggle against fate, fortune, chance and their families or the many barriers that face them to try and make their relationship work. This is what truly makes this play tragic, as we are rooting with them for their relationship to work, and yet the more effort they put in to trying to come up with some sort of solution which would salvage a happy ending from the chaos the more tragedy seems to be imminent.
I don't think there's a major character that doesn't contribute to the tragedy. But I think that the title characters contribute most. They're the ones who make the bad choices that lead to their doom.
As far as ideas go, I think that the idea of romantic love contributes most to the tragedy. Romeo and Juliet die because they have this idea that one should fall in love and marry on that basis. That's unremarkable today, but back in Shakespeare's time high-born people, especially, would have expected to be married off for political, social, or economic (or all three) purposes. The idea (held by Romeo and Juliet) that romantic love is the proper reason for marriage is what really causes the tragedy.
Discuss what theme this character contributes to and how? Provide examples wherever possible