No Adding or Subtracting
The Torah commands us: “Do not add to it” and “Do not detract from it.”
This raises a question concerning the Rabbinical commandments such as lighting Shabbat candles, kindling Chanukah lights and washing hands before partaking of bread. How could the Sages presume to enact new obligations left unmentioned in the Torah?
Maimonides and many others have raised this question. Maimonides’ answer is that the rabbis never claimed that their enactments constituted Biblical commandments, adding to the total number of 613.
Rabbinic Authority
The question still remains: what authority did the rabbis have to be able to add on obligations even if they were not included in the count of Biblical commandments?
The answer is given in this week’s parsha:
The Torah exhorts us:
“…you should act in accordance with the teachings that they [the Sages] instruct you, and according to the judgments they issue to you. You must not divert from the words they tell you, right or left.
The Talmud (Shabbos 23a), in its discussion of Chanukah states that before we light the Chanukah Menorah we must recite the blessing: “Blessed are You, G‑d our G‑d, King of the universe who has sanctified us with His commandments and commanded us to kindle the Chanukah lights.”
The Talmud asks where do we find that G‑d commanded us to kindle Chanukah lights? The answer is the forgoing verse, where the Torah commands us to follow the dictates of the Sages and not deviate from what they tell us, right or left.
Chanukah Paradigm for All Rabbinical Commandments
The Talmudic model for the rabbinical commandments is the lighting of the Chanukah Menorah. It stands to reason that if Chanukah is the model for Rabbinical commandments, it would at least be hinted so in the Torah. Indeed, Chanukah lights are hinted in the foregoing verse that commands us not to deviate from what the Sages tell us, neither to the right or left.
The Talmud teaches us that we light the Chanukah Menorah on the left side of the entrance to our homes, so that the Mezuzah is on the right as we enter and the Chanukah Menorah is on the left.
Thus, when the Torah states that we should not deviate from right or left it is hinting to us: do not deviate from lighting the Menorah on the left, opposite the Mezuzah on the right.
However, there are times when the Chanukah Menorah itself is placed on the right side of the entrance. If, for whatever reason, there is no Mezuzah on the door post (for example, when one rents a house there is no obligation to affix a Mezuzah for the first 30 days), one would then place the Menorah on the right side of the entrance. Hence, the mitzvah of kindling the Chanukah lights is associated with both the right and the left.
Chesed and Gevurah
Upon reflection, the right and the left exemplify all rabbinical commandments, which are primarily “left” oriented but secondarily “right” oriented as well.  
In Jewish mystical thought, the left is associated with the Divine attribute of Gevurah-Restriction. The bulk of rabbinical laws are restrictive; geared to safeguard Biblical laws. The Sages added restrictions on the Sabbath to prevent people from violating the biblical laws concerning work on the Sabbath.
Generally speaking, the rabbis were sensitive to human frailty and realized that it can lead people to transgress unwittingly.  For that reason, they enacted restrictive measures to ensure the integrity of the Torah commandments.
This emphasis on restriction is clearly identified with the left, or G‑d’s power of judgment. 
However, the Rabbis also enacted positive and expansive laws, such as reciting blessings before partaking of food, kindling Shabbos and Chanukah candles, reading the Megillah on Purim, etc. So, while the bulk of rabbinical commandments are geared to preserve the integrity of the Biblical laws - associated with the left - they also represent efforts to increase positive energy into the world - which is associated with the right: the Divine attribute of Chesed-Kindness.
Purim and Chanukah: Right and Left
The Chassidic work Likkutei Yehudah quotes the Imrei Emes, who interprets the left and right in this verse as an allusion to Purim and Chanukah. As we stated earlier, Chanukah is primarily associated with the left. However, the hero of Purim, Mordechai, is referred to as “Ish yemini-a man on the right.”  This is so because he heralded from the tribe of Binyamin, which is a composite of two words, Ben and yamin, a child of the right.
This can shed light on the prayer we recite Friday night, in which we speak of spreading out to the right and the left in relation to Moshiach:
“To the right and to the left you shall spread out, and G‑d you shall extol. And we shall rejoice and exult, through the man who is a descendant of Peretz [Moshiach].”
Hence, we may interpret the words “to the right and to the left” used here as an allusion to the Holidays of Purim and Chanukah. When Moshiach comes, according to one Midrashic source, all the holidays will lose their radiance and will dwell in the shadow of new, more powerful Holidays that will mark the onset of the Final Redemption. However, the Midrash asserts there two holidays will not lose their exalted status in the Messianic Age: Chanukah and Purim, the two rabbinical Holidays associated with the right and the left.
This, then, is what the prayer refers to when it speaks of spreading to the right and the left with the coming of Moshiach, the descendent of Peretz. At that time, we will rejoice and exult. Moshiach will break out of conventional wisdom that ascribes more significance to the Biblical Commandments and Holidays, revealling the more sublime nature of the rabbinical Holidays.
No Less Divine
At this point, we must try to understand why the rabbinical laws and Holidays are indeed greater than the Biblical ones that come directly from G‑d.
In truth, Chassidic thought explains that the rabbinical laws are no less Divine than the Biblical ones. On the contrary! When qualified Torah Sages enact a law, whether it is on the right or on the left, they are actually uncovering G‑d’s hidden Will. The fact that it was not mentioned explicitly in the Torah is not a reflection of its inferior importance.  On the contrary, it is because the law transcends formal codification. Things that are very powerful cannot be easily expressed in words.
Rabbinical laws are actually subsets of the Biblical ones that have eluded conventional means of expression. This can be compared to a person who refuses to share what he or she wants because he or she feels that the wish is so important to the relationship that the other should know it intuitively. The Divinely inspired Sages knew intuitively that this is G‑d’s inner will.
In the Messianic Age, we will truly appreciate the power vested in the Rabbinical laws encompassing both right and left, because they transcend categorization.
Fidelity to rabbinical laws today is one way of preparing ourselves for the Final Redemption, when we will experience unbounded joy in their observance.