"Come between us, good Benvolio; my wits faint." (Mercutio).
"Come, come with me, and we will make short work; For, by your leaves, you shall not stay alone Till holy church incorporate two in one."
After Romeo and Juliet confessed their love to each other, he goes to Friar Laurence the next day and asks if he would marry them. This changes everything with the medieval feud that is occurring with the Montague's and the Capulet's .
On a street, Mercutio and Romeo are talking with Benvolio, while being a tiny bit buzzed. They are joking around and being dirty in a way where it is evident that they are truly good friends. Finally showing and leading to why Romeo stood up for Mercutio when his good pal gets killed by Tybalt.
While Romeo and Juliet wait to get married, they explain and say words to each other that are very sentimental. Because they love each other so much they are willing to do whatever it takes to make sure they are married. Which ultimately leads Friar Laurence to say you musnt' not stay alone until two has turned into one, meaning until they are married.