In the prologue to Shakespeare's Romeo and Juliet, the Chorus sets the scene for the play. In fact, the Chorus reveals the entire plot of the play.

The first part of the Prologue gives the location of the play, introduces the main characters, and provides background information on the relationships of the characters, all in six lines.

Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life ... (Prologue. 1–6)

The pair of lovers is described as "star-cross'd," meaning, on one hand, that they're preordained by the stars to meet, and, on the other hand, that their relationship will be beset by unfortunate circumstances.

These prophecies, in a sense, come true. The pair of lovers, Romeo and Juliet, meet seemingly by accident at a feast held by Juliet's parents, Lord and Lady Capulet. Members of Romeo's family, the Montagues, aren't invited to the feast because of an ongoing feud—the "ancient grudge"—between the families.

Romeo and some of his friends attend the party uninvited, which is how Romeo and Juliet meet, and how they come to fulfill the first meaning of "star-cross'd." The other meaning of "star-cross'd" is fulfilled because the families are feuding, which seriously complicates Romeo and Juliet's continuing relationship, which they both acknowledge in the scene at the feast.

ROMEO. Is she a Capulet?
O dear account! my life is my foe's debt. (1.5.125–126)

JULIET. My only love, sprung from my only hate!
Too early seen unknown, and known too late!
Prodigious birth of love it is to me
That I must love a loathed enemy. (1.5.147–150)

The Chorus's words "take their life" foreshadow the end of the play, but in this context the words mean that the relationship of the "star-cross'd lovers" arises out of the conflict between the families.

In the prologue, the Chorus also outlines the action of the play, and brings Fate—not just the circumstance of Romeo and Juliet's "star-cross'd" life—more fully into Romeo and Juliet's relationship.

Whose misadventur'd piteous overthrows
Doth, with their death, bury their parents’ strife.
The fearful passage of their death-mark'd love,
And the continuance of their parents’ rage,
Which, but their children's end, naught could remove,
Is now the two hours’ traffic of our stage ... (Prologue. 7–12)

"Death" is mentioned twice within two lines, and "their children's end" make it clear that Romeo and Juliet die in the play. "Death-mark'd" implies that fate influences their deaths.

It's important to remember that the Chorus is telling the story in retrospect. The events of the story have already occurred, and the Chorus is relating the events from their own perspective and infusing the story after the fact with the elements of the "star-cross'd lovers" and their "death-mark'd love." The Chorus believes that Fate intervened in Romeo and Juliet's relationship and that the "star-cross'd lovers' were fated to die.

This is where Romeo and Juliet's shared tragic flaws come into focus. No person and no circumstance forces Romeo and Juliet to fall in love or to get married. Aside from falling in love at first sight, which might be attributed to the "stars"—and might just as easily be attributed to their immaturity—Romeo and Juliet's own impulsiveness determines the course of their relationship.

Romeo and Juliet were enabled in their impatience and their impulsiveness to get married by Friar Laurence and the Nurse. Friar Laurence also devised an absolutely terrible scheme to bring Romeo and Juliet back together after Romeo was banished for killing Tybalt. Nevertheless, Romeo and Juliet made their own decisions regarding those events, as immature and impulsive as those choices might have been.

Romeo and Juliet also made their own decisions to kill themselves. There was no compelling reason for Romeo to commit suicide when he went to Juliet's tomb, other than his own romantic, heroic notion of killing himself for love and joining Juliet in death. If Romeo had paused just a few minutes before impulsively drinking the poison, Juliet would have revived from the sleeping potion, and Romeo and Juliet could have hopped on Romeo's horse and galloped away to Mantua to live happily ever after.

There was no reason for Juliet to kill herself, either, other than her own romantic notion of dying for love and joining Romeo in death.

It looks like the stars and Fate might have had something to do with the story of Romeo and Juliet, but that too-simple explanation of the events of the play deprives Romeo and Juliet of their flaws and their humanity. It's much more interesting that Romeo and Juliet live their own lives, make their own decisions, and suffer the unfortunate consequences of their own mistakes.

Juliet's tragic flaw is her impulsivity. On the very night that she meets Romeo and after only two conversations (one at the party and one in the garden), she decides to marry him. When she bids him good night, once they've exchanged lovers' vows, she tells him that she will send someone to carry his message to her tomorrow about "Where and what time [he will] perform the rite" and become her husband (2.2.153). Later, when Juliet knows she'll be forced to marry Parris, she tells the friar that she "long[s] to die" and that he should not take so "long to speak" (4.1.67). She cannot wait, and she wants an answer or advice from him now. Further, when he proposes the idea of faking her own death, she jumps at it, not considering what could go wrong or the potential negative consequences of her choice. Her impulsivity leads to her demise.

It is considered fate that the young lovers take their lives in part because of what the Chorus says before the play's events even begin. Romeo and Juliet are described as "star-crossed lovers" who have sprung from their parents' "fatal loins" (Prologue. 5–6). Moreover, the Chorus describes their love as "death-marked" (Prologue. 9). In act 1, scene 4, Romeo also seems to acknowledge fate. He feels compelled to attend the Capulets' party, though he has a premonition that it may lead to his death. He says,

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 despised life closed in my breast
By some vile forfeit of untimely death.
But he that hath the steerage of my course
Direct my sail (1.4.113–120).

Juliet's tragic flaw is her loyalty to Romeo.  She loves him and is so loyal to him that she could not bear to live without him.  So when he died, she had to die as well--to be forever with him.  She was willing to take the potion that made her appear dead--even though it scared her that it might kill her all because she would not be forced to marry Paris.  Her loyalty was her strength, but it was also led to her demise.

The play begins with the Prologue--and that is where it tells us of their fate.  "Two star-crossed lovers take their life." That theme is present throughout all of Shakespeare's works.  Any mentioning of the stars refers to fate and what has already been decided.  It's as if the characters have no control over their lives or over their decisions and actions. They must accept their fate because they can do nothing to alter it.