When Romeo runs to Friar Laurence to arrange for his plan to marry Juliet, Friar Laurence is at first confused since Romeo was only the day before so love sick over the pretty Rosaline. In fact the Friar actually assumes that Romeo has slept with Rosaline and so has come to confess his sin to be forgiven.
Friar Laurence suggests that perhaps he is only filled with lust and not love as Romeo insists. Romeo makes it clear that he is serious by suggesting to Friar Laurence that they arrange for he and Juliet to be married. This begins to change the Friar's opinion but he still is concerned that Romeo has fallen in love only with his eyes and not with his heart.