Final answer:
Friar Laurence is skeptical about Romeo's feelings for Juliet, believing his affections to be fleeting, given his quick shift from Rosaline to Juliet.
Step-by-step explanation:
Friar Laurence initially finds it difficult to believe that Romeo is serious about Juliet because of how quickly Romeo has switched his affections from Rosaline to Juliet. The Friar assumes that Romeo's feelings for Juliet are fickle and shallow, similar to his previous infatuation with Rosaline. This skepticism stems from the Friar's knowledge of Romeo's recent despair over Rosaline and his sudden declaration of love for another so soon after. The Friar suspects that Romeo is more in love with the idea of love, rather than with Juliet herself.