She fears that she may be tricked into drinking poison by Father Lawrence, or will go mad: "O, if I wake, shall I not be distraught, / Environed with all these hideous fears?" (IV.3). In a Romeo-like frenzy, Juliet finally resolves, having no apparent recourse (other than bigamy): "Romeo, I come! this do I drink to thee" (IV.3).
Juliet becomes more and more heedless over the course of the play, despite her early intelligence and caution, the closer she becomes to Romeo. But Romeo's haste and the change it spawns in Juliet's character is not simply the result of his youth: all of Verona society behaves badly and hastily, as reflected in the actions of the older generation. The servants fight with barely a pretext of an offense and even Juliet's father, Lord Capulet, the oldest character (who should theoretically be the wisest, except for the Friar) also acts impetuously. He betroths his daughter to Paris without asking her, to cheer her up, after her cousin's untimely death. Rather than respecting Juliet's grief (even though unbeknownst to her father, she is really mourning because Romeo is banished just as must as for Tybalt's death) Capulet instead sees marriage as a quick solution to his daughter's unhappiness. "Monday! ha, ha! Well, Wednesday is too soon, / O' Thursday let it be: o' Thursday, tell her, / She shall be married to this noble earl. / Will you be ready? do you like this haste?" he says to Paris, admitting the marriage is taking place in an hasty fashion, very close to Tybalt's death (III.4). Lord Capulet has been pressing Juliet to marry since the beginning of the play, even though she is barely thirteen. By showing how members of the older generation, like Lord Capulet and Friar Lawrence, act hastily and seem to hope that the swift marriages of young people will heal the ills of society, Shakespeare suggests that it was perhaps inevitably that these star-crossed lovers would come to ruin.
The King James Bible, the translation of the Bible that was written around the same time as Romeo and Juliet contains the words: "he that hasteth with his feet sinneth" (Proverbs 2). The 'sinful' or dangerous nature of haste is demonstrated over and over again in the Shakespearean tragedy. Although Juliet and Romeo are the play's only suicides, the other characters in the play who act hastily -- Lord Capulet, Juliet's nurse, and Friar Lawrence -- and do not have the excuse of youth, like the teenage Romeo and Juliet, also create the conditions that orchestrate the title protagonist's premature demise.
"Proverbs." The King James Bible. Bible Gateway November 29, 2010.