In an article for Mattot-Massei 5773, Rabbi Cherki discusses the laws of personal vows. Human speech is a form of mutual communication with G-d.
The laws of "nedarim," making a vow, which include the prohibition, "Let him not break his word, he must do everything that comes out of his mouth" [Bamidbar 30:3], show us the serious character of the sanctity of speech. What a person says he or she must do.
The laws of nedarim are quite surprising. Just by speech alone, a person can create what is in effect a private religion – a personal set of rituals. He can add on personal prohibitions which do not exist in general, negative mitzvot that do not appear in the Torah. He can also add on for himself positive obligations which the Torah did not give him. And all of this takes place as a result of speaking, nothing more. It is even possible to cancel a positive mitzvah. If a person vows not to touch a type of object used for a mitzva he is not allowed to perform the mitzva (Rambam, Hilchot Nedarim 3:36). The only thing that a person may not cancel is a prohibition.
This is quite surprising, since the mitzvot were given to us by G-d, while here we are discussing a mitzva that was initiated by man! The answer is that the Torah wants to teach us that speech is not simply a natural skill but is rather an extension of the speech of the Holy One, Blessed be He. All the mitzvot were given to us orally, from the mouth of G-d. And human speech, which is also divine, can join together with the Torah of G-d and create an innovation, a new mitzva.
This can lead us to a much deeper issue. The relationship between man and the Holy One, Blessed be He, can be thought of as a dialogue, a discussion between the Creator and the created object. We might have thought that the mitzvot are laws of reality, like the laws of nature (gravity, and so on), and that they have no inherent significance. But this is not so – the mitzvot are the word of G-d. Why does this matter? It is impossible to make a compromise with a law of nature, it is totally impersonal, and it is not related to human morality or to whether a person repents or regrets his action. On the other hand, since the mitzvot are the word of G-d, we stand not before an impersonal law but rather before somebody. If we are in contact with somebody, we can speak, show contrition, or even promise that we will never do the same thing again. And this repentance of ours can be accepted. There is indeed a conversation between the Creator and His creatures.
Source: "AS SHABBAT APPROACHES" – a biweekly column in Shabbat B'Shabbato, Mattot-Massei 5773, Volume 1481. (Zomet Institute) See: www.zomet.org.il/eng