What is the Friar's reaction when he is informed Romeo wishes to marry a Capulet?
The Friar's initial reaction is to chide Romeo for his fickle nature. Just a couple of days earlier, Romeo was weeping for Rosaline, but he is now informing the Friar that he is in love with, and wishes to wed, Juliet, who is a Capulet. He was "doting, not loving" Rosaline, and the Friar is fairly certain, despite Romeo's protestations, that the same is true of Juliet.
But Friar Laurence also sees an opportunity in Romeo's newfound love. He hopes that the union of Romeo and Juliet will reconcile their two feuding families. "[T]his alliance," he tells Romeo, "may so happy prove / To turn your households’ rancour to pure love." Thus he makes a fateful decision to marry Romeo and Juliet not in spite of the fact they come from feuding families, but because of it. The audience is aware even at this point that the Friar's decision, however well-intentioned, will not end well for the two "star-cross'd" young lovers. Romeo's love for Juliet is genuine, despite the Friar's initial reaction, but it is not able, at least in life, to transcend the hatred between their two families.