How does the friar change in his attitude toward Juliet and what does this show about his character?

The Friar thinks that rushed love can lead to bad things. Yes, that's a little foreshadowing! Although he is reluctant to marry them so quickly, he thinks that perhaps this union might bring the two feuding families together,

"In one respect I'll thy assistant be;

For this alliance may so happy prove,

To turn your households' rancour to pure love."