In another of Shakespeare's plays, Julius Caesar, Cassius tells the more superstitious Brutus,
Men at some time are masters of their fates:
The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings. (1.2.145-147)
Although Romeo and Juliet are "star-cross'd lovers"; that is, they are in danger being the children of mortal enemies; the young couple make several choices that place them in the way of harm. Thus, their destiny, as the saying goes, is not so much a "matter of chance, but a matter of choice." These dangerous choices of Romeo and Juliet have been a result of their impetuous natures. In Act II, Scene 2, for instance, Juliet insists that Romeo prove his love to be real by pledging to marry her: "If thou doest love, pronounce it faithfully" (l.94).
Later, their marriage complicates, rather than solves, problems as Friar Laurence has hoped because Romeo impulsively slays Tybalt after Mercutio is killed; this action, in turn, causes Romeo's banishment, which in turn puts him in Mantua where there is a plague and Romeo is quarantined. So then, he does not learn what has happened to the unfortunate Juliet, whose father wants her to marry the nobleman Paris. As a result of this predicament, Juliet goes to Friar Laurence who works out the scheme of her feigned death, which later ends in tragedy because Romeo is not informed that Juliet is actually alive and because Friar Laurence abandons Juliet in the catacombs.
In short, the tangled web of tragic events in the lives of Romeo and Juliet is woven by their impulsive acts in the beginning. The young lovers become fated because of their choices, rather than because of the "stars." Their story is a lesson to readers of the importance of making wise choices in their lives, for the results of impulsive decisions can, indeed, be tragic as choices--free will--are often limited by the predicaments in which people have already put themselves.