In Romeo and Juliet, why does Friar Laurence seem reluctant toward Romeo's requests to marry Juliet?

During Act 2 scene 3, friar laurence says "is rosaline that thou didst love so dear, so soon forsaken? ( ...) The sun not yet thy sighs from heaven clears, thy old groans ring yet in mine ancient ears." Basically, he thinks Romeo musnt truly be in love with Juliet if he had just a couple days or so ago been crying over Rosaline not liking him back. He's reluctant towards romeo's requests to marry juliet because he thinks its too soon for romeo to know he is in love with juliet.