The prohibition "He shall not break his word" – one's obligation to safeguard their oaths and vows – enhances the internal desire for norms of personal behavior to an actual commandment. The vow thus transforms into a commandment for the one taking the vow. The actual content of the vow becomes less significant once it is included in the Torah's framework and thus becomes a Torah obligation. Thus, in this instance, the Torah makes man both into the commander and the commanded simultaneously. It allows man to be in the place of the Giver of the Torah (if that were possible) by announcing a system of prohibitions and values that will limit him.
The Gemara discusses the relationship between the Mitzvot in the Torah and what one commands oneself. May one vow to keep a Torah command or to abstain from a Torah prohibition? May one vow to transgress a Torah prohibition? These two questions can shed light on two different but related issues. The difference is the general direction. Vowing to keep a Torah command is strengthening the Torah obligation, while the second possibility attempts to create two contradictory obligatory systems: an independent Torah prohibition and the Torah prohibition to break one's vow. In this way, he creates a conflict between his vow and God's commandment. However, there remains a similarity between the two cases. In both, man is attempting to transform the obligation from a heteronomous one to an autonomous one based on a personal decision. When it comes to his religious world, he wants to be in control by determining priorities and the system of obligations. Both of these questions touch upon the fundamental issue that stems from the very possibility of vowing that puts man in the place of God.
In fact, the basic assumption of the Gemara is that a contradiction exists. The concept "he is already sworn and obligated from Har Sinai" does not allow man to accept an additional source of his obligation to keeps God's word. It is impossible to artificially create a dual system of obligation that will transfer the source of obligation to man himself.
The Gemara states in Nedarim:
"From where do we learn that one may take a vow to fulfill a commandment? As it says 'I swore and I will keep to keep your just laws.'
But he has sworn and is obligated from Har Sinai!?
Rather this teaches us that one is permitted to encourage oneself."
The Gemara learns from the verse that one may duplicate God's commandment and strengthen it through a vow, but one may not create an additional source of obligation. At the very most, one may take a vow in order to encourage one's self to fulfill the Torah obligation. This discussion relates to prioritizing man's vow and God's commandments.
An additional discussion does not relate to the ability to vow but to a situation in which a partial overlap exists between the vow and Torah law. The Gemara discusses a person who vowed not to eat and ate a non-Kosher food item. Is he punished as one who ate a non-Kosher food item or as one who did not keep his vow? Here too the assumption is that his preexisting obligation from Har Sinai relegates the problem of not keeping a vow as not serious. However, as long as the vow is broader than the Torah prohibition, it will be a binding vow applying also to the Torah prohibition.
Thus, one can add a vow to commandments we received at Sinai. Regarding prohibitions the vow is not valid even if it strengthens a prohibition unless it expands beyond the Torah prohibition. A vow against Torah law is null and void. One who takes such a vow has taken an attainable vow because he may not fulfill his vow.
The question of overlapping between a vow and Torah law is a question that is at the source of many conflicts that exists in the life of a believing Jew. To paraphrase Socrates: Is good considered good because God commanded it or did God command it because it is good? Must one recognize the good in God's commandments or is it good because it was commanded? The answer is not unequivocal. One is permitted to develop a specific connection to a commandment based on identification with its details, but the source of obligation stems not from a vow but from an obligation to listen to God.
This is an important lesson and advice for a religious experience that includes excitement and happiness. It is problematic if one does not recognize that religious obligation stems from God, but there is nothing wrong with personal identification and excitement within our worship of God. As the Rebbe from Gur describes in the Sfat Emet (Behalotekha 646):
"How beautiful are thy steps in sandals" (Song of Songs 7,2)
'Steps' refer to the excited hearts of the Jewish People.
'Sandals' refer to the need for that excitement to be limited so nothing inappropriate is let in.
The elders are praiseworthy for having the judgement to limit the excitement and not be out of control."