Friar Laurence agrees to perform the marriage ceremony in Romeo and Juliet for what reason?
The simple answer is that he hopes that by marrying the young Capulet and the young Montague, he will end the feud between the two warring families, an argument that has been tearing the city apart for as long as everyone can remember. It is also the friar's duty, and he appears to hold a great deal of love and hope for the two young lovers in his heart. Farbeit from him to deny these two the opportunity to seal their love for each other in the vows of marriage. The hopeful peace is just an added benefit to the happy union of the two.
Friar Laurence knows that Romeo does not really love Juliet. He says, "Young men's love then lies not truly in their hearts, but in their eyes" (II, iii, 68). He, however, agrees to perform the marriage because he thinks that a marriage between the Capulet and Montague children might end the longstanding feud, saying, "In one respect I'll thy assistant be/For this alliance may so happy prove/To turn your households' rancour to pure love" (II, iii, 89-91). This, of course, does not turn out to be the case.