Juliet falls for Romeo just about as easily as he falls for her. The way they speak to each other with religious undertones explains how serious they are about each other. They use words like "pilgrim," "devotions," and "shrine" to discuss their feelings for one another.
When it is time for all to leave for the evening, Juliet runs to the Nurse immediately to find out who he is. After running back and forth, the Nurse finds out that Romeo is the only son of their enemy, the Montagues. the fact that Juliet goes out of her way to find out Romeo's name and information about him shows how interested she is in him. Earlier she played the perfect daughter who was dutiful and respectful to her parents. Now she is venturing out a little bit on her own, which is a little risky for her in such a time period.
In this scene, Juliet is very receptive to Romeo's amorous advances. Moreover, she reciprocates his attraction and passion. When Romeo claims that he is too unworthy to touch her hand, for example, Juliet immediately returns the compliment by telling him not to be so hard on himself:
Good pilgrim, you do wrong your hand too much.
She also praises Romeo's "mannerly devotion," a further sign that she is receptive to his advances. In addition, the fact that she allows Romeo to kiss her provides further evidence of Juliet's instant love for Romeo.
Juliet's interaction with Romeo in this scene reveals much about her character. She is confident, for example, and enjoys behaving flirtatiously. That Juliet returns Romeo's kiss demonstrates the strength of her feeling towards him, even though they have only just met.
When Juliet finds out that he is a Montague, however, she curses love for allowing her to fall for him. Notice how she does not abandon her love, despite Romeo being related to the only man she claims to hate. This demonstrates Juliet's commitment to Romeo while also showing that she values love above hate.
Juliet truly falls in love with him, just as much as he falls for her. She even goes as far as telling the Nurse to go find out if he is available/married. Of course, she then finds out that he is "the only son of [her] great enemy." Basically, she responds with, great...the one and only man a fall in love with and he just had to be the enemy of my family.