how or why did the conflict happen in act 2 scene 2?

Romeo presents the Friar with a conflict when he appears claiming himself in love with Juliet only hours after mooning over Rosaline. Love for the sake of love proabably wouldn't have shocked the Friar, but Romeo's desire to marry Juliet immediately does concern him. None-the-less, the conflict is overcome when the Friar agrees to marry the couple, as he believed the alliance between the Capulets and Montagues might serve to end the feud between the families.