If you’re talking about his vow in the balcony scene, then Juliet felt overwhelmed and unsure about his vow. Juliet didn’t want their love to be too rushed, she wanted to take her time. In act 2 scene 2, Romeo says “Lady, by yonder blessed moon I vow, that tips Witt silver all these fruit tree tops”, to which Juliet replies “O, swear not by the moon, the inconstant moon. Hat monthly changes in her circled orb, lest thy love prove likewise variable.” She wants them to get a good sleep to think over their relationship and revisit it in the morning.
