"Asu syag laTorah - make a fence as a safeguard around the Torah laws" is the instruction given to us by the Anshei Keneset Hagedolah (Avos 1:1). Chazal (Yevamos 21a) derive this obligation to protect the mitzvos by enacting rabbinic decrees from the last passuk in Parshas Achrei Mos (Vayikra 18:30), "ushmartem es mishmarti". While gezieros d'rabanan are found in all areas of halacha, a unique status was assigned to the safeguards protecting prohibitions of gilui arayos (prohibited relationships). We are taught (Avos D'rabbi Nosson 2:1) that the Torah itself enacted safeguards to protect us from violating the prohibitions of arayos. All physical contact is prohibited by the Torah itself. These prohibitions are to be a fence to protect us from committing actual gilui arayos.
Chazal extol the value of sayagim around mitzvos. Shlomo Hamelech is praised by Hashem for instituting gezeiros in the realm of carrying on Shabbos. Chazal (Eruvin 21b) consider this to be one of the greatest achievements of Shlomo Hamelech. Why are gezeiros drabanan so critical? Why is Shlomo Hamelech praised so lavishly for this gezeira that it even overshadows his other accomplishments, such as building the beis hamikdash?
The Ramban (Shemos 20:8) comments that all the negative commandments of the Torah are rooted in yiras Hashem. In contrast to the positive commandments which serve to express our ahavas Hashem, one expresses one's fear and awe of Hashem by refraining from what He prohibits.
In the realm of positive mitzvos one can express one's ahavas Hashem to different degrees. One who merely fulfills mitzvos in the basic form without embellishing on the beauty of their performance has only attained a certain level of ahavas Hashem. Performance of a mitzvah in the most beautiful way possible clearly demonstrates, and serves as a tool to enable us to grow in, our ahavas Hashem.
In the realm of mitzvas lo taase it is more difficult to differentiate between different levels of yiras Hashem. Isn't yiras Hashem exhausted by simply refraining from issurim? Where is there room for growth in abstaining from aveiros? Perhaps the key to growth in yiras Hashem can be found in the requirement of "asu syag laTorah". A person who not only refrains from prohibitions, but also creates safeguards that distance him from violating the word of Hashem demonstrates his appreciation for yiras Hashem. One who does not have such safeguards, even if he technically does not violate any prohibition, clearly is lacking in yiras Hashem.
There are two distinct aspects of yiras Hashem. The elementary level is yiras haonesh - fear of punishment. The Rambam in Hilchos Teshuva emphasizes that everyone must begin with this basic level of fear. Hopefully, one's yiras Hashem will mature and reach the level of yiras haromemus - awe for the majesty of Hashem. We allude to these two levels of yiras Hashem in the tefillah for mevarchim hachodesh. We ask Hashem for many things during this tefillah. The only request that we repeat is our desire for a life full of yiras Hashem. Why do we repeat? Apparently we are asking for two distinct types of yiras Hashem. First we request yiras shomayim v'yiras cheit. Yiras cheit is the basic fear that cheit carries with it a punishment. When we beseech Hashem for yiras shomayim v'yiras cheit we are requesting that we refrain from cheit because of yiras haonesh. We then progress to requesting ahavas Torah v'yiras shomayim. This is a very different type of yiras shomayim. This is a yirah that stems from an appreciation of the greatness of Hashem. Just as ahavas Torah emanates from an appreciation of the beauty of Torah, so too this yiras shomayim of yiras haromemus comes from a realization of the absolute awesome power of Hashem.
Asu syag laTorah is the mechanism to demonstrate our yiras Hashem. It serves to indicate both yiras haonesh as well as yiras haromemus. One who truly views cheit as a spiritual poison and understands the severity of onesh involved with violating the word of Hashem will not suffice to passively abstain from aveiros. He will actively search for ways to guard himself from coming anywhere near cheit. Just as one who has poison in his house will not leave it out in a way that it may inadvertently be eaten, one who views cheit as spiritual poison will make every effort to protect himself from any association with it.
Yiras haromemus is the corollary of shivisi Hashem l'negdi tamid. One who truly believes he is constantly in the presence of Hashem will be filled with the awe that is natural to be felt being in His presence. One who is in the king's palace is on his best behavior and scrutinizes his every action and word, lest he offend the king. If we are truly in the presence of Melech Malchei Hamelachim, how much more so are we obligated to guard ourselves not to violate the word of The King. We have to take extra precautions not to even come close to violating an actual mitzvah.
Shlomo Hamelech accomplished many great things during his lifetime. Part of his legacy to us is Asu syag laTorah. .he taught us how to grow in our yrias haonesh and our yiras haromemus. It is through our dedication to gezeiros drabanan that we demonstrate to Hashem our desire for yiras shomayim. May we merit that Hashem grants all of us chayim sheyesh bohem yiras shomayim v'yiras cheit, chayim sheyesh bohem ahavas Torah v'yiras shomayim.