One of the motifs of Shakespeare's Romeo and Juliet is that of the Caution Age vs. the Violence of Youth, despite the rashness of the behavior of the older characters, at times. In Act II, Scene 6, for instance, Friar Laurence acts as the voice of this motif in his response to Romeo's rash love:
These violent delights have violent ends,
And in their triumph die, like fire and powder
Which as they kiss consume. The sweetest honey
Is loathsome in his own deliciousness
And in the taste confounds the appetite.
Therefore, love moderately, long love doth so,
Too swift arrives, as tardy as too slow. (2.6.9-15)
Thus, youth is impetuous and too exhuberant, even violent, while age deliberates and moves more cautiously and prudently. Clearly, the tragic downfall of Juliet is due to her impetuous and violent behavior. For, early in the drama, Juliet is not so rash; she tells her mother that she is not interested in marrying Paris, but, she also says that she will obey her mother and consider him:
I'll look to like, if looking liking move.
But no more deep will I endart mine eye
Than your consent gives strength to make it fly. (1.3.101-103)
However, after she meets Romeo, Juliet soon abandons her tendency to consideration; while she urges Romeo in Act I, Scene 5, not to kiss her lips so quickly and in Act II, Scene 2, to not swear his love by the "inconstant moon," she soon abandons her better judgment as she is "too quickly won" and becomes admittedly foolishly affectionate. Before Romeo departs from the orchard, Juliet swears her love for him, also with the suggestion of violent love: "Yet I should kill thee with much cherishing" (2.2.197).
Later, in Act III after Juliet is told that she must marry Paris, she reacts with violent intent. She is angered at the nurse, who encourages her to go ahead and marry Paris, even though she is aware that Juliet is already married; Juliet calls her "Ancient damnation! Oh, most wicked fiend! (3.5.246), and flees to the cell of Friar Laurence considering suicide if all else fails:
I'll to the Friar, to know his remedy.
If all else fail, myself have power to die. (3.5.252-253)
When she confers with the priest, Friar Laurence suggests a plan: She can feign her death which will give her parents sorrow, but they will be so elated by her return to them that they will forgive her rashness in marrying Romeo. In the meantime, he will notify Romeo about the actions being taken. As fate would have it, however, Romeo does not get the message and, instead, he discovers a Juliet he believes has died. His impetuous behavior leads him to suicide so that he can join Juliet in death. Shortly after he dies, Juliet awakens, and refuses to leave with the timorous Friar, who flees alone, leaving her to discover Romeo's body. With violence rather than rationality and prudence, Juliet reacts to the sounds of a watchman, stabbing herself with Romeo's dagger and dying tragically from her impetuosity and violence.
Juliet's love or perceived love for Romeo is her tragic flaw. She is committed to being with Romeo and sacrifices everything to this end. If a tragic flaw is a reason that someone experiences sadness and pain, then Juliet's tragic flaw would have to be her love for Romeo. Had she not been in love with Romeo, much of what she suffers goes away. Her plans to leave her family, her drinking the poison, and her eventual death are the result of her love for Romeo.
When confronted with the moment of truth in which she must contemplate living with or without Romeo, Juliet concludes that it is better to die than to live without her Romeo. It is in this where one can see that her tragic flaw is her affection for Romeo. If one wished to make this even more poetic, then Juliet's tragic flaw is love. While the issue can be debated if Juliet is in love as opposed to infatuation, she believes it to be love and acts upon it as if it is love. For this reason, identifying Juliet's tragic flaw results in seeing love as the cause of her suffering and basis for her condition of tragedy.