Why does Juliet not want Romeo to swear by the moon?

In William Shakespeare's "Romeo and Juliet," Juliet asks Romeo not to swear his love by the moon because she believes that the moon is inconstant and changeable. She says, "O, swear not by the moon, the inconstant moon, / That monthly changes in her circled orb, / Lest that thy love prove likewise variable."

Juliet is suggesting that the moon's changing phases represent the fickle nature of love. Just as the moon changes from full to new and back again, so too can a person's feelings change over time. She fears that if Romeo swears his love by the moon, his love for her may also be subject to change.

Juliet's mistrust of the moon is also rooted in her belief that the moon is associated with witchcraft and magic. In Shakespeare's time, the moon was often seen as a source of mystery and danger. It was believed that the moon could influence people's emotions and behavior, and that it could be used to cast spells or curses. Juliet's fear of the moon reflects the common superstitions of her time.

By asking Romeo not to swear by the moon, Juliet is expressing her desire for a love that is constant and unchanging. She wants Romeo to love her regardless of the circumstances, even if the moon is waxing or waning. She is searching for a love that is eternal, not one that is subject to change or influenced by the whims of fate.