Shakespeare's Act IV, Scene IV of Romeo and Juliet serves the dramatic purpose of creating dramatic irony. There are several different types of irony. Dramatic irony refers to moments when the audience, or readers, understand something beyond what the characters themselves understand. The irony in this scene is that, now that Juliet has consented to marry Paris, her parents are bustling about, laughing and chatting gaily with the servants, as they prepare for Juliet's wedding day tomorrow. However, they don't know what we, the audience knows, that Juliet has made arrangements with Friar Laurence to fake her own death. While her parents are laughing and gay now, ironically, we know that soon they will be miserable.
Act IV, Scene IV also stands in great contrast with Act III, Scene V. In this earlier scene, while believing that he wants what is best for his daughter, Lord Capulet has threatened to disown Juliet should she refuse to marry Paris. Not only that, when Juliet remains steadfast in her refusal, Lady Capulet declares, "I would the fool were married to her grave," meaning, "I wish [Juliet] were married to her grave," or dead, for disobeying her parents (III.v.143). Further dramatic irony can be seen in this line, while Lady Capulet is unaware, she has actually prophesied exactly what will soon happen to her daughter, which intensifies the dramatic irony seen in Act IV, Scene IV. Now that Juliet has finally relented and consented, her parents are now happy with her again and joyously preparing for her wedding day. We especially see her parents' happy mood in their playful bantering in this scene. For example, when Nurse scolds Lord Capulet for not retiring to bed, warning that he'll get sick if he stays up "watching" the proceedings, he replies that he's watched over things all night long before for even less significant reasons. To this, his wife wittily replies:
Ay, you have been a mouse-hunt in your time;
But I will watch you from such watching now. (IV.iv.12-13)
Here, Lady Capulet is making a joke, implying that her husband's real reasons for wanting to stay up is so that he can watch the maids at work. The phrase "mouse-hunt" can be translated to mean "woman chaser," showing us her meaning (eNotes). She next says that she "will watch [him] from such watching" (13). This line actually makes a pun out of the word "watch." Both Nurse and Lord Capulet used "watch" to mean "staying awake," but Lady Capulet is using it to mean "keep an eye on him while he looks at other women" (eNotes). Hence, this sort of witty repartee and banter helps portray their merry mood, which is ironic, considering that Juliet fakes her death next, leading to her actual death.