Juliet's attitude towards love is very much determined by her age and lack of experience. It could be said that love, for her, is initiated by physical attraction and led by her emotions. She falls in love with Romeo, most likely, because she is physically attracted to him at first. One of her first comments (before she even knows his name) to Romeo is "You kiss by the book" (ie: "You are a great kisser"). This seems to be a shallow connection, but at only thirteen years old, what else could Juliet possibly know about love and relationships, aside from her physical and therefore immediate emotional feelings?
Unlike Romeo, however, the audience gets a glimpse into Juliet's mind in the balcony scene, where we see that she is not entirely unthoughtful about love and her current situation. She considers the fact that Romeo is from the family of her sworn enemy. She considers back and forth how quickly this romance could go (and laments, even with Romeo, that it is moving too quickly). Though she is only thirteen, she does have a concept of some of the consequences of her feelings. But, like any other teenager, her rational side is overcome by her physical desire, and in the end she is persuaded to go forth with a very immature and risky relationship in a very hasty manner.