The friar is surprised at Romeo's unusually early visit. He suspects that Romeo must have had a troublesome sleep, or did not sleep at all, to be awake and seeking him out so early. He states, in part:
Therefore thy earliness doth me assure
Thou art up-roused by some distemperature.
Romeo assures the friar that he has had a pleasant sleep, which further confounds the priest. He asks whether Romeo spent the night with Rosaline, who he believes is still the object of Romeo's affection and the root of his problems. Romeo tells him that he has forgotten about her and the pain she brought him.
The friar is quite pleased by this reply, but wants to know where Romeo spent the night. Romeo speaks in riddles by saying that he's been "feasting with mine enemy" and that he is wounded but bears no hatred. Laurence tells him to speak more plainly, and Romeo informs him that he has fallen for Juliet Capulet, a member of the family with which the Montagues are feuding. He states that they have declared their love for each other and asks for the friar to marry them that same day.
Friar Laurence is aghast at Romeo's sudden change of heart and admonishes him by stating that he had been counselling the lad for all the troubles he had with Rosaline, who did not return his affections. Romeo often tearfully spoke to him about his great love for Rosaline, but now he has inexplicably forgotten about her. He ends by stating that women need strong men to sustain them, implying Romeo is not displaying such strength.
Romeo is offended by the friar's remark, and reminds him that the friar often scolded him for loving Rosaline, to which the friar replies that he admonished him about his infatuation, which is not love. When Romeo tells him that the friar told him to bury his love, the friar retorts that he did not mean that he should permanently bury love itself, but rather that he should replace one love with another.
Romeo pleads that the friar should not be too critical, as Juliet requites his love while Rosaline did not. The friar then accedes that Rosaline knew that Romeo knew only the basics of love (that's why she rejected him), and then instructs the love-struck young man to accompany him. Friar Laurence will prepare for their betrothal because he believes Romeo and Juliet's marriage might bring happiness to the two warring families and turn their hate into love.
Romeo is in a hurry to get things done, but the friar tells him to slow down, since those in a rush easily stumble (and fall). These words are quite ironic and add another dramatic element to the play. The friar's plans tragically backfire later.
The friar is shocked by Romeo's news that he is going to marry Juliet. The friar states, "Wisely and slow; they stumble that run fast" (II, iii) a line that shows the hasty nature of Romeo's behavior. It is this haste that will be part of Romeo and Juliet's downfall. The friar is confused, in part, because Romeo was JUST head over heels for another girl, Rosaline. However, by the end of the scene, the friar agrees to marry the couple, and the reality the friar would benefit from joing the oldest feuding families in Verona cannot be ignored. Thus, the friar's motivation is in question as well.
In this scene (especially at first), Friar Laurence seems to think that Romeo is making a fool out of himself with regards to love.
He knows that Romeo has felt like he is in love with Rosaline and has spoken to Romeo about being more moderate in his feelings. When he now finds out that Romeo has forgotten about Rosaline and wants to marry Juliet, he feels, perhaps naturally, that Romeo is being a bit hasty and rash.
By the end of the scene, however, Friar Laurence has consented to marry the two, hoping to end the feud between their families.
Ultimately, he is happy for both Romeo and Juliet as it presents an opportunity for the feud between the two families to end.