One admonition Friar Laurence gives Romeo in Act 3, Scene 3 is to warn him that his ungratefulness is a "deadly sin." When Romeo wails that being banished from Verona is "purgatory, torture, hell itself," in other words, worse than death, Friar Laurence refers to his lack of gratitude and lack of perspective as, "O deadly sin! O rude unthankfulness!" and reminds him that for killing Tybalt, according to the law, he deserves death.
Another admonition Friar Laurence gives Romeo reminds him that committing suicide is a sin. He does so by commanding Romeo to "hold thy desperate hand," when Romeo draws his sword, pointing it at himself. He further admonishes Romeo by telling him that he is acting like a woman and not like a man, crying womanly tears and making as much use of reason as animals do. Also, through the line, "Fie, fie, thou shamest thy shape, thy love, thy wit," Friar Laurence admonishes Romeo to be ashamed of himself.
His final admonition to Romeo encourages Romeo to begin feeling thankful and happy. Friar Laurence reminds Romeo that "Juliet is alive," that Tybalt would have killed him, but instead he "slew'st Tybalt," and finally that the "law that threaten'd death becomes thy friend and turns it to exile." Due to all of these things, Romeo should be happy, not wailing in sorrow and threatening suicide.
Romeo is distraught in this scene because he has been banished from his beloved and his city. He wails to the friar that he wishes for death instead of banishment. The friar criticizes him for being so ungrateful. The Prince changed the law for Romeo's case, making the penalty for murder banishment instead of death. "O deadly sin! O rude unthankfulness! Thy fault our law calls death; but the king Prince, Taking thy part, hath rushed aside the law, And turned that black word 'death' to 'banishment.' This is dear mercy, and thou seest it not."