"Affliction is enamoured of thy parts, and thou art wedded to calamity"
Romeo And Juliet, III.3
Here's another aspect of Friar Laurence which has always puzzled me: when Romeo comes in and tells him that he's fallen in love with Juliet (without even mentioning her name: he merely refers to her as "the fair daughter of rich Capulet"), the Friar says nothing about the potential danger of this relationship. He somewhat mercilessly chides and mocks Romeo for having dropped, overnight, his infatuation with Rosaline for a new love, and then, in the blink of an eye, agrees to marry them, apparently with the sole intention of reconciling their families. From the outside, this decision has always struck me as a little rash (especially for the Friar), and from the inside, until just recently, I found it very difficult as an actor to motivate.
However, I believe I have figured it out
In Act I, Scene 2, when Romeo is reading off the guest list for the Capulet's feast, it includes "my fair niece Rosaline". Which means (obviously) that Romeo starts the play in love with a Capulet, and the Friar already knows all about Rosaline. This explains why the Friar doesn't warn Romeo of the dangers of falling in love with the enemy: they've already been through all that, probably a hundred times. (It's interesting that Benvolio thinks the best place for Romeo to get over his obsession with Rosaline, a Capulet, is at a Capulet feast populated by more Capulet women. Perhaps Romeo would have been better off checking out the action at a Montague mixer.)
This also explains why the Friar is so quick to marry the two lovers with the hope of ending the feud: he has been thinking about the possibility, and perhaps even formulating the plan, since Romeo came to him with the great news that he had fallen in love with Rosaline. Of course, the fact that Rosaline "hath sworn that she will still live chaste" and has refused Romeo's advances tells the Friar that this particular relationship is not the great love story to melt the hearts of the feuding families. But when Romeo insists that Juliet "doth for grace for grace and love for love allow", Friar Laurence is convinced he's got the winning ticket.
Shakespeare pretty much establishes that Friar Laurence and Juliet are familiar with one another, and given all of the Friar's subsequent action in the play, it's not difficult to imagine that he sees in her what she seems to inspire in everyone she meets: as her father puts it, "she is the hopeful lady of my earth". As for Romeo, despite his adolescent propensity for the occasional hormonal histrionic, "Verona brags of him to be a virtuous and well govern'd youth". Indeed, after the play's opening brawl, Capulet himself sees hope for peace between the two households.
In this context then, and given his faith in a god that (presumably) wants peace, it's difficult to condemn the Friar too harshly for believing that, by wedding the two young lovers, he has found the solution for finally ending the town's "ancient grudge".
Of course, in a way he could never have imagined, and would never have desired, he has.