Romeo and Juliet are talking in the famous balcony scene. Juliet says to Romeo 'wherefore art thou' on line 36 of act 2 scene 2. by this she is saying 'why must you have been born a Montague, because if you where a Capulet, than we could be in love without any problems.
During scene 4, Mercutio and Benvolio are discussing Romeo. he is still crazy over love, as thou shot by Cupid's arrow. they think thats about Rosaline, although it is about Juliet. In addition, Tybalt sent a letter to Romeo challenging him to a duel.
In the last scene of the act Romeo and Juliet get married. Friar Laurence says to Romeo 'these violent delights have violent ends.' wishing that he won't act to brash. He is suspicious of Romeos speed of switching partners and warns him that it could end as explosively as it began.