why is Friar Laurence concerned about Romeos request to marry Juliet?

The Friar believes that Romeo's request and love for Juliet are impetuous, that Romeo's feelings are fickle and unreliable. Regardless, though the Friar is surprised that Romeo has forgotten Rosaline so quickly, he is nonetheless delighted, because Romeo and Juliet's union presents an opportunity to quell the raging feud between the Montagues and Capulets.