After Romeo and Juliet meet, kiss, and speak briefly, Juliet asks her Nurse to find out what his name is. While the Nurse leaves her to ask, Juliet says,
If he be married,
My grave is like to be my wedding bed. (1.5.148-149)
In other words, she says that if the Nurse tells her that he is already married, then Juliet will never marry anyone, ever. Her wedding bed will actually be her grave. This is a pretty impulsive statement, as Juliet is only thirteen and she has only just met Romeo, and yet she is willing to make such a dramatic statement.
When the Nurse returns and tells Juliet that he is the son of her family's enemy, Juliet says,
My only love sprung from my only hate! (1.5.152)
Again, Juliet's impulsivity leads her to make a statement that implies she will never love another: Romeo will be her "only love," despite her youth and the very early stage of their relationship (if we can even call it a relationship at this point!). She believes herself to be utterly in love after having only exchanged a few words and kisses with a young man, and, moreover, she cannot imagine that she will ever love again.
Later, on her balcony, Juliet speaks to herself, believing that no one else is around to hear her. In part, she says, addressing her remarks to Romeo (who she does not realize is there),
[...] be but sworn my love,
And I'll no longer be a Capulet. (2.2.38-39)
Here, again, after having only exchanged a few words and kisses, and despite her knowledge that loving Romeo would be totally unacceptable to her family, Juliet is ready to completely leave her family and join his. If he is willing to accept her love, she is impulsively ready, now, to give up her family and her life to join his.