Answer:
Juilet doesn't want Romeo to swear his love by the moon becuase she believes the moon is always changing and she wants something permenat
Step-by-step explanation:
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.
(ll,ii,113-117)