When the Friar first discovers that Romeo has fallen in love with Juliet, and the young man asks for his help, he expresses reservation, but ultimately agrees:
In one respect I'll thy assistant be;
For this alliance may so happy prove
To turn your households’ rancour to pure love.
In other words, the Friar hopes to reconcile the houses of Monatgue and Capulet, whose feud has torn Verona apart, through bringing the two young lovers together. One one level, his plot is an utter disaster, claiming the lives of Romeo and Juliet in the end. In another sense, however, it is successful. Montague and Capulet, grief-stricken, do in fact agree to renounce their feud in the end. Still, the ultimate tragedy of the play is that it cost the lives of Romeo and Juliet (and Paris, Tybalt, and Mercutio) to make it happen.