Devarim 13:1 instructs the Jewish people to be careful neither to add nor subtract from the commandments. This is the second time (see 4:2) over the course of less than ten chapters that the Tora formulates such a demand. The Vilna Gaon explains the need for two verses dealing with this theme by associating the first with the prohibition against completely eliminating or adding an entire commandment, and understanding the second as a warning regarding the performance of one of the 613 in an altered unauthorized manner. When it is recognized that there are some practices which while included within the Tora, e.g., Nedarim (vows) in general (Devarim 23:23) and the state of being a Nazirite in particular (BaMidbar 6:14), are nevertheless frowned upon in the Tora text, innovating religious categories that are nowhere to be found in the Tora would certainly be problematic.
The verses instructing us to adhere precisely to Tora commandments beg the question as to why someone might ever think that it would be permissible to tamper with the totality of the Mitzvot of the Tora. The very nature of each individual commandment would appear to contain within it the implicit obligation to conform to what has been ordered, regardless of one's personal opinion regarding the appropriateness or meaningfulness of what s/he has been asked to do. Within a military context, failure to obey an order from a superior officer generally leads to significant negative consequences; God certainly qualifies as the Jewish people's Commander-in-Chief at all times and in all places. Aren't we therefore always "on duty" and completely subject to any and all commandments which we have been given?
Sephorno accounts for the impetus to ignore certain Mitzvot by positing that when one thinks that the reason why the commandment was originally given does not currently apply. Since God obviously would not want us to do something which was inherently absurd, we obviously are exempt. The commentator (following R. Yitzchak's approach in Sanhedrin 21b) notes that such an attitude is exemplified by Shlomo HaMelech, who despite the three specific negative commandments that are given to Jewish kings in Devarim 17:16-17, i.e., he is not to have too many horses, wives and/or too much wealth, felt that as long as he did not cause Jews to live in Egypt in order to obtain these horses, did not allow his numerous wives to adversely affect his religious observances, and kept a proper perspective regardless of the wealth that he would personally acquire in his royal capacity, he could honor these commandments in the breach. Not only was Shlomo incorrect in terms of his general approach, when he presumed that it was permissible for any intelligent individual to consider him/herself to be exempt from at least some of God's Commandments due to an idiosyncratic analysis of why s/he is confronting extenuating circumstances, but David's son apparently also failed to understand his own personal weaknesses and vulnerabilities, as reflected in Melachim Alef 10:26-11:8.
The biased and therefore unreliable rationalism that the Sephorno worries will lead individuals to justify omitting commandments is attributed by R. Shmuel David Luzatto (ShaDaL) to underlie the drive to add to Mitzvot as well. ShaDaL suggests that, for example, were one to begin to sacrifice animals not specifically authorized by the Tora on the Tabernacle/Temple alter, such as the deer and the antelope, perhaps others will come to eventually justify and engage in human sacrifice.
While the decision not to perform a commandment can usually be attributed to the need to assert one's independence and autonomy vis-à-vis God by striving for a modicum of freedom from Divine Obligation, a desire to change the commandments by adding to them, may appear, at least at first glance, as something positive, a manifestation of the pursuit of performing the Mitzva in a better and more spiritually meaningful manner. But, as is obvious in ShaDaL's example, when the individual loses sight of the fundamental values that Judaism advocates, as in the case where one fails to recognize how human sacrifice is an abomination in God's Eyes, "the road to hell can be paved with good intentions." The only assurance therefore, that one has not to draw false conclusions regarding the purpose of the commandments leading to improper observance, is to carry all of them out completely and precisely, without alterations, both additions and subtractions being deemed unacceptable.
A more theological explanation for the directives in Devarim 4:2 and 13:1, arises in light of the rule of R. Chanina, quoted in Kiddushin 31a, Bava Kama 38a, 87a, and Avoda Zora 3a: :Greater is the one who is commanded and fulfills the commandment, than the one who is not commanded, and (nevertheless) fulfills the commandment. When an individual takes upon him/herself a religious practice that is not mandatory for him/her, the action is accompanied by the suspicion that perhaps the decision to voluntarily undertake this action has more to do with the individual's personal pursuit of meaning, than his/her seeking to carry out God's Will. On the other hand, when one performs a religious ritual precisely and for no other reason than the belief that this is the Will of God, a statement has been made more about the person's relationship with the Divine, than any feelings about him/herself. Consequently, altering even a mandatory commandment to make it more precisely conform to one's own desires and understandings would appear to at least potentially shift it from the category of something that was commanded, to an action that is merely optional and the result of some type of need for self-expression.
Perhaps Rabban Gamliel Beno Shel R. Yehuda HaNasi put it best, when he said (Avot 2:4), "Make His Will like your will, so that He Will Make your will like His Will." The perceived clash between the desires, understandings, and objectives of each of us, including the great Shlomo HaMelech, and the components of the lifestyle that God Demands of us, may arise because we have insufficiently internalized not only the rationale of a specific commandment, but also the overall implications of what it means to be a true Eved HaShem (servant of God.) Were we able to achieve a greater level of Hitbatlut (self-diminishment), then all sorts of Mitzvot that presently appear to be beyond our ability and interest, would suddenly make so much more sense and would become so much more do-able. The whole of the Tora is greater than the sum of its individual Mitzvot, regardless of the amount of adding and subtracting in which we may try to engage.
As Ellul and Yomim Noraim approach, it is time to strive to perceive and realize spiritual wholeness, by diminishing the adding and subtracting of observances that disrupt our becoming all that we can be Jewishly.
Shabbat Shalom!