Romeo had just met Juliet a few hours earlier. He is so in love that he snuck back into the castle to see her. He says if a Montague can't marry a Capulet, then he wasn't going to be a Montague.
Henceforth I never will be Romeo.
Juliet was getting impatient, waiting for the Nurse to get back with Romeo's instructions for marriage. When the Nurse got back, she tried to stall telling Juliet by saying she is tired and out of breath.
How art thou out of breath, when thou hast breath To say to me that thou art out of breath?
Romeo is asked Friar Lawrence to marry Romeo and Juliet. The Friar agrees to marry them, but is surprised. He says how Romeo was just obsessed with Rosaline a couple hours ago.
Holy Saint Francis, what a change is here! Is Rosaline, that thou didst love so dear, So soon forsaken?