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Even a tragedy needs some comic relief, and who better than Juliet's bawdy, lower-class nurse? It's comic gold: she's a lower-class women, so that's already funny; and she's a nurse, which means all she can talk about are bodies—bodies having sex, bodies having babies, bodies nursing babies. Not to mention, she has a real way with the dirty jokes, like this one:
I must another way,
To fetch a ladder by the which your love
Must climb a bird's nest soon when its dark. (2.5.77-79)
Here, she's literally talking about getting a ladder for Romeo to climb up so he can spend the night in Juliet's bedroom. But there's never just one level with the Nurse—To "climb a bird's nest" is also slang for having sex. It's a laugh a minute.
Until it's not.
The Nurse and Juliet may have a loving, teasing sort of relationship at the beginning of the play, but when Juliet needs her most—after her parents order her to marry Paris—the Nurse betrays her. Romeo is as good as dead, the Nurse tells Juliet, and she had better forget him and marry Paris. So, is the nurse as responsible for Juliet's death? Is she going to be one of the people "punished," as the Prince says at the end? Maybe. We have two questions to settle. Let's start with the easy one:
(1) Why does the Nurse help Juliet hook up with Romeo?
This is easy, because … we don't know. We see Romeo and Juliet on the balcony and hear the Nurse calling her in; we see Romeo with Friar Laurence; and then we cut straight to the scene where the Nurse shows up as Juliet's messenger. What happens in between there? Is the Nurse just playing the role of "bawd"—a woman who pimps out a young girl? It's possible. Is she genuinely trying to do what's best for her charge? Maybe. Or is she somehow trying to angle personal gain for herself? All we can do is speculate.
Here's a juicier question:
(2) Why does the Nurse betray Juliet by telling her to marry Paris?
Well, maybe she really does believe what she tells Juliet:
Romeo is banished, and all the world to nothing
That he dares ne'er come back to challenge you,
Or, if he do, it needs must be by stealth.
Then, since the case so stands as now it doth,
I think it best you married with the County.
O, he's a lovely gentleman!
Romeo's a dishclout to him. An eagle, madam,
Hath not so green, so quick, so fair an eye
As Paris hath. Beshrew my very heart,
I think you are happy in this second match,
For it excels your first, or, if it did not,
Your first is dead, or 'twere as good he were,
As living here and you no use of him. (3.5.226-238)
Maybe the Nurse simply doesn't understand that Juliet's love for Romeo is the real thing, and not some childish infatuation. (Although in that case, why would she encourage the marriage?) If you're feeling a little judgmental, you could say this attitude is both callous and unperceptive. Her dirty-minded—or low-class—way of looking at love cannot comprehend a love like Juliet's.
There's also the possibility that the Nurse doesn't want to lose Juliet to an uncertain future with Romeo in Mantua. Selfishness might play a role in wanting her beloved Juliet to stay in Verona and marry Paris—and doubtlessly bring the Nurse with her when she moves to Paris's house. Either way, this is a pretty cruel move.
On the other hand, maybe the Nurse does understandJuliet's love for Romeo—she's just woman of the world and knows how limited Juliet's options are, in a way that an idealistic little teenager doesn't. After all, she does try to stand up to Lord Capulet when he is yelling at his daughter, a bold move: "You are to blame, my lord, to rate her so," the Nurse tells him (3.5.177).
In response, Lord Capulet attacks her verbally—and perhaps physically as well. So the Nurse just gives up, which may have something to do with Lord Capulet's violent thrashings. Only then does the Nurse decide that Juliet has to marry Paris. In this interpretation, the Nurse's praise of Paris is just a way of facing the facts. She knows Juliet's love for Romeo is real, but in order to save Juliet from the disastrous consequences of her secret marriage, she tries to make a second marriage to Paris seem acceptable.
So: is the Nurse a crude opportunist—or a loving realist? Is she on Juliet's side—or is she only on her side?