In Act I of Romeo and Juliet, Juliet changes from disinterest in the idea of marriage to Paris in Scene 3 to falling in love with Romeo in Scene 5.
This quick switch in Juliet's perspective exemplifies the theme of impetuous behavior and passion.
First, when her mother asks Juliet if she might consider Paris, who is a cousin to the Prince, as a husband, Juliet expresses her disinterest:
I'll look to like, if looking liking move,
But no more deep will I endart mine eye
Than your consent gives strength to make it fly. (1.3.99-101)
Ironically, Juliet's disinterest seems to reflect that of her father, Lord Capulet, who in Scene 2 has told Paris that his child "is yet a stranger in the world" and too young for marriage, although he does not discourage Paris from wooing her and get[ting] her heart."
Then, in Scene 5 after Juliet meets the impassioned Romeo, at first she pushes back from his attempts to kiss her, but then allows him a kiss because as she later declares in this last scene of Act I when the Nurse informs her that Romeo is a Montague,
(aside) My only love sprung from my only hate!....
Prodigious birth of love it is to me,
That I must love a loathèd enemy. (1.5.138-141)
Juliet, who has expressed no interest in marriage or love earlier, meets Romeo and falls in love with him in only one scene.