This is because she had gone to him right after her Nurse had told her to marry Paris. She is outraged by her only confidant's choice, and because of that she does not think in a calm, level-headed manner. The friar changes her fate when he gives her the vial, as he is giving her a way out of the marriage to Paris. Juliet's hope of Romeo's fate changing does come true, but not in the way she intended when she says,.
"O Fortune, Fortune, all men call thee fickle. .
If thou art fickle, what dost thou with him.
That is renowned for faith? Be fickle, Fortune,.
For then I hope thou wilt not keep him long,.
But send him back" (3.5.60-4).
What she means by this quotation (on the last night her and Romeo spend together, as he is slipping into the darkness) is that she wants Fortune to be inconsistent like it has been before, so that Romeo's fate can change and he can come back to her. This does happen, as mentioned above, but when Romeo goes back to her, he thinks that she is dead and his overload of passion for her makes him kill himself. Lastly, Juliet seals her own fate by stabbing herself with a dagger when she wakes up and sees him dead beside her. Her excessive passion for him makes it seem impossible for her to live without him. As one can see, Fortune not only helps convey that excessive passion makes one do unsuspecting things, but it also shows that it is dangerous to act out in pride or rage.
The sonnet form helps convey the idea that the excess of passion will make one do crazy, unexpected things, as well as how acting out in hubris or rage is very dangerous. Firstly, Romeo and Juliet follows a structure similar to that of a sonnet: there is a volta at Act III Scene I where Mercutio and Tybalt are killed. This scene shows Tybalt acting out in hubris and Romeo acting out in rage and excessive hatred. In a sonnet, the volta signifies the change in the mood. In Act III Scene I, the play's entire genre shifts from romance to tragedy.
