In this scene Romeo wants to change his name for Juliet to prove his love. They end up declaring one anothers love for each other and promise to get married the next day at nine in the morning.
What shall I swear by?
Do not swear at all. Or, if thou wilt, swear by thy gracious self, Which is the god of my idolatry, And I'll believe thee.
Romeo need Friar Lawrence to marry him and Juliet in secret because no one was allowed to know, and Friar was surprised about this because he believed that romeo was still in love with Rosaline. He ends up marrying the two teenagers because he believes it can bring both families together.
Amen, amen. But come What sorrow can, It cannot countervail the exchange of joy That one short minute gives me in her sight. Do thou but close our hand with holy words, Then love-devouring death do what he dare, It is enough I may but call her mine.