Juliet is the picture of steadfast love and loyalty to Romeo during the short time they are together. She sticks by him even after he kills her beloved cousin Tybalt, although her first instinct is to curse him.
As he is about to leave for exile to Mantua after their wedding night in act 3, scene 5, Juliet shows that her heart will be with him at all times:
Art thou gone so? love, lord, ay, husband, friend!
I must hear from thee every day in the hour,
For in a minute there are many days ...
In other words, she wants them to stay close and stay in touch despite the distance that will be between them. This is all the more poignant, as she is the one who urges him to leave as dawn breaks, again in loyalty, as she doesn't want him discovered and killed.
Later, Juliet states definitively that she is on Romeo's side after the Nurse urges her to forget about Romeo, deny the marriage ever happened, and marry Paris, who offers her a comfortable life. This is the shrewd advice of a practical woman who been forced to get along in the world. However, Juliet rejects this advice, berates the Nurse for her hypocrisy, and repudiates her, say she will no longer confide in her:
Thou and my bosom henceforth shall be twain.
From here on in, Juliet will turn only to the Friar for advice about how to avoid marrying Paris.