Romeo and Juliet are already married, but the slaying of Tybalt by Romeo has interrupted their happiness. Romeo feels that he shouldn't have let Mercutio fight his battles for him. He kept trying to make peace because Tybalt was now related to him and he didn't want to fight him because of Juliet. He considered Tybalt to be family.
Knowing that Romeo has been banished to
Mantua, Friar Laurence promises that he will find Romeo's manservant and that he will make sure that Romeo receives updates on all the good things that are going on in Verona. (Act 3, Scene 3)
Reference: The Literature and Language Book, by McDougal Littell
Romeo has just gotten the friar's promise to marry he and Juliet. The friar has failed to consider the disastrous consequences of agreeing to carry out this promise.
Romeo blames his preoccupation with Juliet and his love for her for the death of Mercutio. He realizes that his more masculine tendencies have been lacking as of late, and because of this, he has let Mercutio down. Romeo feels he should have been the one fighting, not calling for the fight to stop, and his love for Juliet is the reason for it, "O sweet Juliet,/Thy beauty hath made me effeminate/". He feels that he has become feminine in his love for Juliet, yet after Mercutio's death, he returns to his more masculine thoughts of revenge and honor.