Thanks! this is a brilliant answer =) x
Juliet doesn't actually appear in Act 3 Scene 4 as Romeo has just killed Tybalt, and her parents think she is locked away in her room mourning her cousin's death. In fact, she is spending her wedding night with her husband before he is sent into exile.
Act 3 Scene 4 shows us Paris coming to woo Juliet. Earlier in the play, Paris approached Lord Capulet for Juliet's hand in marriage. At that time, Lord Capulet asked Paris to take two years and woo Juliet so that she would fall in love with Paris and they could be married happily in love. By the time this scene ends, this idea seems to be forgotten.
In Act 3 Scene 4, Juliet refuses to come down to visit with Paris, and Lord Capulet decides that he will marry Juliet to Paris on Thursday when today is Monday. He says that his daughter is ruled by him and will be obedient to him. Lord Capulet comments that this wedding could pull everyone out of their saddness after Tybalt's death. Paris is thrilled with Lord Capulet's change of heart and agrees that Thursday would be a great day for his wedding.
This scene is filled with dramatic irony as we as an audience know that Romeo and Juliet are already married and another marriage will be impossible. We as an audience also may find ourselves very frustrated with Lord Capulet at this point as he seemed reasonable earlier in the play when he viewed his child as "a stranger to the world" and wanted Paris to woo her for two years. Now he is throwing her into a marriage she would in no way want whether she be married to Romeo or not.
Lord Capulet's motives are unclear at this time. He may be doing this to "cheer up" his family as he states, but he may also be panicking that if Juliet isn't available to be wooed Paris may lose interest and move on. Lord Capulet doesn't want this to happen as he and his family view Paris as a perfect match for Juliet.
This scene adds to the idea that Romeo and Juliet are doomed by fate in that they have a night together, and Friar Lawrence vowed to come up with a plan to help them be together, but another obstacle has been put in their way.