One of Shakespeare's points is that the very young couple actually has no concrete or rational reasons for being in love beyond mutual physical attraction. Physical attraction is their reason for loving each other, and we see both Romeo and Juliet speak of physical attraction frequently.
We learn through Romeo's lines the very first time Romeo sees Juliet that physical attraction is the reason he fell in love with her. In these lines, Romeo spends a great deal of time praising her beauty, even comparing her beauty to a bright object, like a flaming torch or a "rich jewel" hanging against dark skin (I.v.46, 47). He even directly equates love with physical attraction when he asks himself, "Did my heart love till now? Forswear it, sight! / For I ne'er saw true beauty till this night" (54-55).
Juliet even associates her love for Romeo with his physical attractiveness later on. After Romeo slays her cousin Tybalt, she goes through a moment of crisis because she feels she has been deceived by Romeo. She feels deceived because his handsome exterior has led her to believe that he is just as beautiful on the inside, but now it seems to her that his handsome exterior houses a villainous soul. The fact that she feels deceived by his handsome exterior proves that she based her feelings of love for him solely on looks alone. We see her feelings of deception expressed in the paradoxical lines:
O serpent heart, hid with a flow'ring face!
Beautiful tyrant! fiend angelical!
O, that deceit should dwell
In such a gorgeous palace! (III.ii.76-88)
In this passage, the words "flow'ring face," "beautiful," and "gorgeous palace" all serve to show that Juliet's love for Romeo is based on his physical looks. In addition, the words "tyrant," "fiend," and "deceit" show us just how deceived she now feels by his looks. However, Juliet doesn't allow herself to continue to feel deceived by Romeo. She eventually convinces herself to think well of Romeo, speak well of him, and honor him simply because he is her husband. This is the one moment when Juliet allows her love to grow and extend beyond physical attraction. At this moment, her love becomes based on faith of his good character rather than just on what she believed to be true of him due to his looks. Nevertheless, she still does not have any concrete reasons to decide to continue to trust him. Rather, she loves him because she has faith that she should.