'Tis torture and not mercy. Heaven is here, Where Juliet lives, and every cat and dog And little mouse, every unworthy thing, Live here in heaven and may look on her, But Romeo may not. More validity, More honorable state, more courtship lives In carrion flies than Romeo. They may seize On the white wonder of dear Juliet’s hand And steal immortal blessing from her lips, Who even in pure and vestal modesty, Still blush, as thinking their own kisses sin. But Romeo may not. He is banishèd. Flies may do this, but I from this must fly. They are free men, but I am banishèd. And sayst thou yet that exile is not death? Hadst thou no poison mixed, no sharp-ground knife, No sudden mean of death, though ne'er so mean, But “banished” to kill me?—“Banished”! O Friar, the damnèd use that word in hell. Howling attends it. How hast thou the heart, Being a divine, a ghostly confessor, A sin-absolver, and my friend professed, To mangle me with that word “banished”?
Why does Romeo say these lines?
Note the alliteration in "white wonder" that helps emphasise the symbol of Juliet's hand as representing her innocence. You also might like to note the use of onomatopoeia in words such as "howling" that help to convey the extent of Romeo's sadness, grief, anger and frustration.
Actually, this passage is an extended metaphor in which Romeo compares exile to being dead. It is beyond death, where even the carrion flies will have nothing to do with him. To be banished is to no longer exist. No one may speak his name, see his face, touch his hand. He will be nothing.
Heaven is also a recurring symbol. To Shakespeare, Heaven would have been a real place. Romeo is using symbolism and a metaphor to say that Verona is like Heaven because Juliet is there.
Another literary device used in this section is a pun. Romeo says, "Flies may do this, but I from this must fly." He is making a play on the word fly and its double meaning.
Romeo says these lines in response to being banished. The priest is discussing his current situation with him. Romeo is deeply distressed at having to leave his new bride and love behind. The Priest wishes Romeo to see the blessing in banishment verse death.
I agree. Juliet's hand is a recurring symbol. To Romeo, it represents her beauty and her purity. He imagines her hand as soft and perfect. It is her womanly nature. In his mind, her hand is perfect and so is she. Once he has that hand, he has her and he has everything.
This part of the passage demonstrates parallel structure:
More validity, More honorable state, more courtship lives In carrion flies than Romeo.
Taking a look at this line, we see the symbol of innocence demonstrated by the color white:
On the white wonder of dear Juliet’s hand.