One of the major premises of Romeo and Juliet, by William Shakespeare, is that Fate (or the Elizabethan version of it, Fortune) is more in control than any of the characters are. The definition of the word "fate" is that which is out of human control.
The opening lines of the play suggest that these "star-cross'd lovers" will lose their lives, a reference to the alignment of the stars--or fate. Romeo references fate when he says in Act III,
This day's black fate on more days doth depend,
This but begins the woe others must end.
The implication, of course, is that what happens in life depends on fate. Juliet also references fate in Act II when she begs fate to bring Romeo back to her quickly:
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.
She seems to believe that Fortune (fate) is in control, and she also recognizes the capriciousness (fickleness) of fate.
When characters in this play do try to manage, control, or even outsmart fate, the result is deadly. When the young lovers marry without informing their parents (to avoid their parents' expected disapproval of their union), they are tempting fate because it clearly does not seem that they should be together based on their circumstances.
When Friar Lawrence tries to circumvent fate (and death), the result is two deaths. The Prince reverses his proscribed punishment and banishes Romeo, Romeo arrives at the tomb just a moment too early, Juliet wakes up from her death-like sleep just a moment too late...these are all manifestations of things seemingly out of human control.
Too many times, at least in literature, characters try to avoid their fates and are penalized for it. It seems that this is true for Romeo and Juliet.