Why does Juliet reject romeos offer to swear the truth of his love by moon?

Juliet does not reject Romeo's offer to swear the truth of his love by the moon. In fact, she encourages him to do so, saying, "O, swear not by the moon, the inconstant moon, / That monthly changes in her circled orb, / Lest that thy love prove likewise variable." She is simply warning him that the moon is a fickle symbol and that his love may change as quickly as the moon's phases.