Romeo says that "it is the east and Juliet is the sun." Since the sun rises in the east, we can assume that Romeo is making a comparison between the rising sun and his rising affection for Juliet. He is comparing her to something bright and burning just like his feelings for her.
Romeo continues his comparison when he says that if her eyes were in the heavens with the stars, they would "shame those stars as daylight doth a lamp." We know this comparison continues the comparison because he says that if her eyes were in heaven, birds would begin to sing thinking it was morning instead of night. Thus continuing the comparison of Juliet to a rising sun.