Key to questions such as this is understanding the context of specific lines. Romeo and Juliet have just met after Romeo's killing of Tybalt and his subsequent banishment. They have had one night together, and now Romeo must leave before the terms of the banishment come in to play and he can be killed for being in Verona. As they take their leave from each other, they recognise the uncertainty of the future that is facing them. After Romeo has left, Juliet personifies "Fortune," and addresses him, saying:
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.
Of course, one of the key themes in this excellent tragedy is the way in which fate or fortune interferes with our plans. Romeo and Juliet are described at the very beginning of the play as "star cross'd lovers," and fortune is refered to throughout the play. Fortune here is personified as being almost like a fickle, capricious child that acts in an unpredictable way. Juliet here almost prays to Fortune, asking him to return Romeo to her as quickly as possible.