Fate is one of the predominant themes of the play. As the analysis here at eNotes indicates, Shakespeare places relies heavily on the "Elizabethan concept of Fortune or Fate."
For example, "upon learning of Mercutio's death, Romeo exclaims, "This day's black fate on more days doth depend, / This but begins the woe others must end" (III.i.119-120). In Act III, scene v, Juliet addresses Fortune (or Fate) and implores its aid.
O Fortune, Fortune, all men call thee fickle;
If thou art fickle, what dost thou with him
That is renown'd for faith? Be fickle, Fortune:
For then I hope thou wilt not keep him long,
But send him back.
Fortune (or fate) is fickle: it is improbable that so perfectly matched a couple should be barred by a senseless family conflict (the cause of which is never mentioned and cannot be recalled); it is only through mischance that Romeo takes his own life in the mistaken belief that his love is dead, causing her to follow suit."