"but soft! what light through yonder window breaks? it is the east, and juliet is the SUN" - im sure your teacher has talked about the light vs. dark imagery. so by saying that juliet is the sun, hes referring to light, while later when he says "arise, fair sun, and kill the envious MOON" which is referring to dark. however, juliet says that she will not kill the envious moon, for the moon has many phases. she fears that romeos love for her will change just as the moon changes phases. romeo is trying to say that he loves her, but he is not thoroughly thinking about what he is saying.