How does Friar Laurence initially react when Romeo tells him of his plans to marry Juliet?

Friar Laurence is initially skeptical about Romeo’s love for Juliet.

Friar Laurence knows Romeo well. When Romeo tells him that he is in love with Juliet, his first reaction is to wonder if Romeo is sincere. After all, Romeo was supposedly in love with Rosaline only days earlier, and now he says he is in love with Juliet. He seems to be changing his mind very quickly.

When Romeo says he is in a good mood because he got something sweeter than rest, Friar Laurence immediately assumes he has been with Rosaline.

FRIAR LAURENCE

God pardon sin! wast thou with Rosaline?

ROMEO

With Rosaline, my ghostly father? no;
I have forgot that name, and that name's woe.

FRIAR LAURENCE

That's my good son: but where hast thou been, then? (Act II, Scene 3) 

Romeo acts like his being with Rosaline is a completely silly idea. He is in love with Juliet now. Friar Laurence chides him for switching girls so easily:

Holy Saint Francis, what a change is here!
Is Rosaline, whom thou didst love so dear,
So soon forsaken? young men's love then lies
Not truly in their hearts, but in their eyes.
Jesu Maria, what a deal of brine
Hath wash'd thy sallow cheeks for Rosaline (Act II, Scene 3)!

Friar Laurence thinks Romeo is just a young man guided by rampaging hormones. Romeo has to convince him his love for Juliet is sincere, and that he actually wants to marry her. He will never think about Rosaline again.  

One of the reasons Friar Laurence decides to marry Romeo and Juliet so quickly is that he thinks getting a Montague and Capulet together will be good for both families. If Romeo and Juliet can end their parents’ feud, everyone will benefit.