In "Romeo and Juliet," Act 2 Scene 3, why does the friar agree to marry the two young lovers? (line 90-92)

In
Act 2, Scene 3,asksto secretly marryand him.  At first, the friar does not want to perform the
weddingrightly so, too, because the day before Romeo had told Friar Laurence how sick he was
over the fact that his dear Rosaline would rather be celibate than be with Romeo.  

Thou and these woes were all for Rosaline:
And art thou
changed?

Romeo convinces the friar that he is no longer
in love with Rosaline, because he is very, truly in love with Juliet.  

Then plainly know my heart's dear love is set
On the fair
daughter of rich Capulet:
As mine on hers, so hers is set...