As neither Romeo nor Juliet have classical tragic flaws, their deaths are much the outcome of the hatred of their families and the caprice of Fate.
Certainly the love/hate polarity dominates the entire action of the play. In fact, Romeo's and Juliet's awareness of this polarity is evinced early. Having witnessed the aftermath of the hostilities in the streets of Verona, Romeo remarks prophetically, "Here's much to do with hate, but more with love" (1.1.175) while after inquiring of Romeo's name, Juliet acknowledges this polarity, "My only love sprung from my only hate!" (1.5.138).
The conflicts arising from the love/hate relationships involving Rome and Juliet reach their climax in Act III. This love/hate polarity clearly complicates Romeo's position regarding the fiery Tybalt. For, when Tyblat says,
Romeo, the love I bear thee can afford
No better term than this: thou art a villain.(3.1.60)
and Romeo tries to explain that now he loves Tybalt
Tybalt, the reason that I have to love thee
Doth much excuse the appertaining rage
To such a greeting. Villain am I none.
Therefore farewell. I see thou knowest me not. (3.1.61-64)
his statement serves only to enrage Tybalt the more, effecting his attack upon Mercutio after Romeo tries to intervene. But, fatefully, Romeo's intervention is what causes Mercutio to be mortally wounded. So, his new love has brought about a newer hatre, and Romeo avenges his friend's death by killing Tybalt, an act which reignites the feud.
Juliet's life, too, is also greatly complicated by this love/hate polarity since her father insists that she marry Paris when she has already secretly married Romeo. Thus, she must find some solution to her dilemma and runs to Friar Laurence who devises the plan to feign Juliet's death so that the parents will reconcile their hatred when she "returns" to them. However, this plan goes awry by certain acts of Fate.
Fate casts a pall over the actions of the two young lovers, dooming them from the start. Certainly, it is chance that Paris would desire to marry Juliet at the same time that she falls in love with Romeo. Likewise, it is unfortunate that Tybalt and Mercutio argue when he happens upon them; it is, indeed, an act of fickle Fate that Mantua is closed because of a plague and that Friar Laurence cannot get word to Romeo that Juliet lives in Act V. And, even though Romeo cries, "Then I defy you, stars!" (5.1.24), his chance discovering of Juliet just before she awakens is a victory for Fate. When she awakens to find her Romeo dead, Juliet no longer wants to live, also succumbing to whims of the "stars," making hers and Romeo's a "death-marked love."