During the balcony scene in Romeo and Juliet why does object to swearing on moon?

In the famous balcony scene in Romeo and Juliet, Juliet does not object to swearing on the moon. Instead, Romeo is the one who hesitates to swear by the moon, fearing that it is too inconstant and changeable to be a reliable oath. He suggests swearing by something more ثابت and eternal, such as their love.