Why does Juliet tell Romeo not to "swear by the moon" in Romeo and Juliet?

Juliet tells Romeo not to swear by the moon because it changes.  

After the ball, Romeo sneaks into the Capulet orchard to get a look at Juliet.  There, he watches her before she knows he is there.  She sees him, and they engage in some wordplay. She asks if he loves her, and he offers to swear by the moon.

ROMEO

Lady, by yonder blessed moon I swearThat tips with silver all these...

Juliet tells Romeo not to swear by the moon because it changes.  

After the ball, Romeo sneaks into the Capulet orchard to get a look at Juliet.  There, he watches her before she knows he is there.  She sees him, and they engage in some wordplay. She asks if he loves her, and he offers to swear by the moon.

ROMEO

Lady, by yonder blessed moon I swear
That tips with silver all these fruit-tree tops--

JULIET

O, swear not by the moon, the inconstant moon,
That monthly changes in her circled orb,
Lest that thy love prove likewise variable. (Act 2, Scene 2)

Juliet doesn't want him to swear by the moon.  Like the sun, the moon goes up and down.  It is also, as Juliet notes, in a different place based on the month and season.  Therefore, when Romeo swears by the moon, Juliet chides him and tells him not to swear by it. 

Confused, Romeo asks her what he should swear by if not the moon.  She tells him not to swear at all, and if he has to swear, he should swear by himself.  She would believe him then, because she idolizes him.  Thus, the love story begins.