Question

What does Juliet ask Romeo to swear his love by?

Answer 1

Juliet asks Romeo to swear by himself. she also tells him not to swear his love by celestial bodies like the moon because they are variable as they rotate and revolve.

Answer 2

Juliet has promised truth in love and asked for truth in love. Romeo, to prove his truth, begins to swear his by the moon which shines down upon them, but Juliet interrupts him and tells him not to swear by the moon, because it is changeable. He asks what he shall swear by, and she tells him to swear by himself. He begins again, but only gets as far as "If my heart's dear love -- " (2.2.115) before she interrupts again, and tells him not to swear at all. Apparently she feels that for him to swear his love would make everything just too good to be true. She says,

Well, do not swear. Although I joy in thee,
I have no joy of this contract to-night:
It is too rash, too unadvised, too sudden;
Too like the lightning, which doth cease to be
Ere one can say "It lightens." (2.2.116-120)