Basically it's because Romeo tried to swear and make a promise on the moon, and she's saying that's not a good idea because it's always changing in either size, shape, or position. It's just not reliable, especially not when you're trying to tell someone that you love that you mean something so deeply. It make it seem like the promise is quite temporary or not as strongly built together as it should be.
Good luck, rockstar! i hope all is well, and you pass! (:
