From the very start of the play, readers are told that Romeo and Juliet are “star-crossed lovers.” How does the concept of fate/destiny...

Fate is one of the major themes in Shakespeare's Romeo and Juliet. He  announces it in the Prologue:

From forth the fatal loins of these two foes
A pair of star-crossed lovers take their life

Since the term "star-crossed" has to do with astrology and hence fate, Romeo and Juliet will be the victims of a pre-determined universe. The couple's fate is foreshadowed a number of times. Romeo nods to his destiny in his aside in Act I, Scene 4:

I fear too early, for my mind misgives
Some consequence yet hanging in the stars
Shall bitterly begin his fearful date
With this night’s revels, and expire the term
Of a despisèd life closed in my breast
By some vile forfeit of untimely death.

He, is in fact, seemingly resolved to death as he again talks of it just before he marries Juliet in Act II, Scene 6:

Do thou but close our hands with holy words,
Then love-devouring death do what he dare,
It is enough I may but call her mine.

Friar Laurence even concurs with Romeo's premonition:

These violent delights have violent ends

Mercutio joins this chorus in his dying scene in Act III, Scene 1. Although he makes jokes about his own demise, he is not jesting when he wishes ill fortune on the Montagues and Capulets:

Ask for 

me tomorrow, and you shall find me a grave man. I

am peppered, I warrant, for this world. A plague o’
both your houses!

Juliet sees Romeo as though he is in a tomb when they part ways after their honeymoon in Act III, Scene 5:

O God, I have an ill-divining soul!
Methinks I see thee, now thou art so low,
As one dead in the bottom of a tomb.
Either my eyesight fails or thou lookest pale.

Shakespeare uses a series of dramatic ironies throughout the play and so Romeo and Juliet are the victims of several misunderstandings and miscommunications:

• Romeo and Juliet meet and fall madly in love before they know the other's identity.


• Neither Mercutio nor Tybalt realizes that Romeo and Juliet are married so they don't understand why he backs down to Tybalt in Act III, Scene 1 and tells Tybalt that he actually loves him.


• Lord Capulet knows nothing of his daughter's marriage to Romeo so he promises her to Count Paris, thinking it will prove a happy day for the family after the untimely death of Tybalt. He even thinks Juliet is crying over Tybalt when he breaks the news about Paris. She is actually crying about Romeo being banished.


• Paris thinks all is well after bumping into Juliet at Friar Laurence's cell. She minces words but he is "sure" that she loves him.


• Lord Capulet is happy when Juliet bends to his will and agrees to marry Paris. Little does he know that she plans on faking her death with a potion mixed by Friar Laurence.


• The Nurse, Lord and Lady Capulet as well as Paris all believe Juliet is dead when she is discovered unconscious in her room the day she was to marry the Count.


• Balthasar has no idea of the Friar's plan for Juliet to fake her death and so promptly reports the news to Romeo who has actually dreamed that he was revived by Juliet's kisses.


• Count Paris doesn't know that Romeo was married to Juliet so apprehends him for a "villain" at the tomb. Romeo kills him.


• Even though Romeo comments that Juliet still looks very much alive and that "death’s pale flag is not advanced there" he still takes the poison only a few moments before Juliet is revived and the Friar comes into the tomb.

Fate and luck are two sides of a double edged sword and so luck conspires against Romeo and Juliet:

• Romeo bumps into the illiterate servingman in Act I, Scene 2 and discovers Rosaline will be at Capulet's party. Benvolio urges him to go. He falls instantly in love with Juliet when he first sees her. 


• Tybalt overhears Romeo talking about Juliet and becomes enraged, leading to the challenge which will play out in Act III, Scene 1. 


• Mercutio is not able to avoid Tybalt's sword point as Romeo gets between them and Mercutio suffers a mortal wound.


• Rather than fleeing Verona, Tybalt comes back to the scene and thus is challenged by Romeo who kills him.


• The Friar is not only a priest but also a chemist and so is able to mix the potion which Juliet drinks to avoid marrying Paris.


• Lord Capulet moves up the wedding day and the Friar has no chance of reaching Romeo after Friar John is delayed with the original message by a plague threat in Verona. Romeo has no idea of the Friar's plot.


• There happens to be a poor apothecary in Mantua who is willing to sell Romeo poison despite it being illegal in Mantua to sell the drug. 


• Romeo arrives at the tomb just as Paris is paying his last respects to Juliet, which costs Paris his life.


• There is no poison left on Romeo's lips so Juliet has to stab herself with a dagger.


• Lady Montague's grief over Romeo's banishment "stopped her breath."