Why does Paris want to marry Juliet so soon?
We never learn explicitly why Paris wants to marry Juliet so soon. In Act 1, Scene 2, Juliet's father tries to persuade him to wait and pursue in her two more years, when she'll be nearly sixteen. But Paris says, "Younger than she are happy mothers made" (1.2.12). But Capulet feels that marrying too young can be detrimental to a girl, so he encourages Paris to court her and try to win her heart. Her feelings, he says, will certainly play a role in whether or not he gives his consent Paris's suit later on.
Later on, after Tybalt's death at Romeo's hands, Capulet is willing to broker the marriage between his daughter and Paris because he wants to pull her out of her mourning and give the family a happy occasion to celebrate. He proposes the following Thursday, just three days away, and Parris replies, "My lord, I would that Thursday were tomorrow" (3.5.32). Even a few days away seems like too long for him to wait! Perhaps he simply knows what he wants and doesn't need more time to think. Perhaps he wants to snatch Juliet up before some other lover presents his suit. We really never know for sure why he's in such a hurry.