In Romeo and Juliet, why is the moon described as envious?

Romeo compares Juliet to the sun, and says the moon is envious because Juliet is prettier.

Romeo is quite lovestruck. He seems to believe in love at first sight, and is caught up in appearances. He is interested in Juliet for her looks more than her personality. He never really gets to know her for who she is. He is only attracted to her beauty.

Romeo sees Juliet at her family's party, and is fascinated...

Romeo compares Juliet to the sun, and says the moon is envious because Juliet is prettier.

Romeo is quite lovestruck. He seems to believe in love at first sight, and is caught up in appearances. He is interested in Juliet for her looks more than her personality. He never really gets to know her for who she is. He is only attracted to her beauty.

Romeo sees Juliet at her family's party, and is fascinated with her beauty. He asks her name and doesn’t even care that she is a Capulet, meaning his family feuds with hers. As a Montague, he should not be interested in her. Juliet's parents will not let Romeo be with her, and Romeo's parents will not approve, either. 

Romeo doesn’t care. He follows Juliet, and then watches when she is at her window. He comments on her beauty, comparing her to the sun.

But, soft! what light through yonder window breaks?
It is the east, and Juliet is the sun.
Arise, fair sun, and kill the envious moon,
Who is already sick and pale with grief,
That thou her maid art far more fair than she (Act II, Scene 2).

In Romeo's metaphor, he compares her to the sun because he thinks she is lovely and she warms him. He feels like nothing can compare to her. Everything else revolves around her. Romeo reasons that when the sun comes up, the moon goes down because it is jealous of the sun. The sun is too beautiful and the moon just has to leave. By comparing Juliet to the sun, Romeo is saying all the other girls might as well go because Juliet has them all beat.

Although this famous speech is beautiful, it is also superficial. Yes, Romeo does love Juliet, but he loves the image of Juliet that is in his mind more than Juliet herself. Romeo and Juliet never have a chance for a normal relationship, so they will never know if they actually would have loved each other.