When Juliet speaks to Romeo from her balcony, she professes her love for him, but she expresses hesitation for overhasty promises due to the suddenness of his appearance and the tenuous nature of his secretive visit to enemy territory. In lines 116–122 of Act 2, Scene 2, Juliet sums it up thusly:

Although I joy in thee,

I have no joy of this contract tonight,

It is too rash, too unadvis’d, too sudden,

Too like the lightning, which doth cease to be

Ere one can say it lightens. Sweet, good night!

This bud of love, by summer’s ripening breath,

May prove a beauteous flow’r when next we meet.

Juliet uses two metaphors in this segment: she compares his promise of love to lightning* because it has been made so quickly. And she compares his love to a bud, which needs time to reach the full beauty of a bloom. 

But although she's telling him to slow down, they end up moving quickly to marriage, and in less than a week, both will be dead. Her comparison of their love to a flower has an ironic double meaning, because summer flowers are a temporary beauty; they fade less quickly than lightning, yes, but they fade nevertheless, showing their beauty only briefly during their short lifespans. These metaphors are both appropriate to the play at large because their love, like lightning and wildflowers, encompasses a few moments of splendor followed by nothingness.

*Juliet's comparison uses the word "like," making it a simile. Many students think that metaphors and similes are opposites or separate categories. In fact, similes are a type of metaphor, so this example fulfills the requirement of the assignment.

One of the best metaphors in Act 2, Scene 2 can be seen in Romeo's opening speech. In his third line, he compares Juliet to the sun in the line, "It is the East, and Juliet is the sun!" (3). Since the sun is bright, radiant, and glorious, the metaphor serves to illustrate Juliet's beauty as equally radiant and glorious. The metaphor serves to appropriately characterize Romeo as one who is not only captivated by Juliet but also fixated on physical beauty.

A second appropriate metaphor can be found later in the scene. After Juliet says, "'Tis almost morning. I would have thee gone--" (189), an extended metaphor is drawn likening Romeo to a pet bird whom she wishes she could "pluck" back into her palm with a "silk thread," keeping him prisoner the way a "wanton," or spoiled child would. In other words, Juliet is likening Romeo to, and wishing he was, a prisoner. The metaphor is further extended when Juliet says that if he were her pet bird she "should kill [him] with much cherishing" (197). In other words, her affection shown through hugs and petting and kissing would be so overwhelming for Romeo as a little bird that she might smother him to death. Likening Romeo to a prisoner pet bird is very appropriate in how it captures the love the couple shared. Because the couple was divided and suffered a great deal of sorrow, their love felt more like a prison than an uplifting element. Not only that, the further extended metaphor of likening Romeo to a smothered pet bird also appropriately captures both their feelings toward each other and their pending doom. Romeo did indeed die as a result of the love he felt and received from Juliet.