Friar Lawrence is surprised when Romeo says he loves Juliet, because he was so recently in love with Rosaline, but he agrees to marry Romeo and Juliet to potentially end the feud.
Friar Lawrence is a confidante and mentor to Romeo. He is aware of Romeo’s love life, and knows that he was pining for Rosaline.
Holy Saint Francis! What a change is here!
Is Rosaline, that thou didst love so dear,
So soon forsaken? Young men's love then lies
Not truly in their hearts, but in their eyes. (enotes etext p. 46)
Friar Lawrence believes that Romeo has been up all night with Rosaline, and is relieved when he learns Romeo has not sinned with her. Although he is surprised that Romeo is in love with Juliet, he seems to think that there is no stopping him, and it is better to just go ahead and marry the two of them.
In one respect I'll thy assistant be;
For this alliance may so happy prove
To turn your households’ rancour to pure love. (p.47)
Friar Lawrence seems to think that Romeo and Juliet will do something hasty if they are not married soon, and he also sees potential in their marriage. He hopes that it will turn their households’ hate into love.
Friar Laurence is, as befitting a priest, worried about the state of Romeo's soul. When he sees that Romeo is tired from a sleepless night and knows that Romeo has been in love with Rosaline, he worries that Romeo has committed the sin of fornication with her. When Romeo assures him that this is not the case, he has little time to savor his relief, for he now discovers to his surprise that Romeo claims to be in love with Juliet.
Friar Laurence is worried about the sudden way in which Romeo has switched his affections from one woman to another. He is concerned that such fickle behavior does not suggest deep-seated love but mere physical lust, which is also sinful.
Nonetheless, due to his affection for Romeo and his desire to heal the feud between the two families, he agrees to marry the young lovers.