This week's parsha features one of the strangest mitzvot in the Torah. The Torah tells us, "Be careful to perform all that I command you today, do not add to it and do not detract from it." It is obvious why we may not detract from the Torah's commandments. That would mean that we didn't fulfil the mitzvah! However, why can't we add? If someone is inspired, why can't he add a fifth box onto his tefillin or another fruit to the Lulav and Etrog? The answer lies in the two different ways to violate this prohibition. We can break it individually by adding elements to a known mitzvah. Alternatively, we can transgress the ban on adding mitzvot by creating new mitzvot from scratch, by supposing that the (sometimes good) thing we are doing is a Torah command.
The one prohibition, the mere addition of components to an existing mitzvah, indicates a need for more appreciation for the role of the mitzvah. By adding to it, we change it; we make it into something else. It is no longer what we were commanded to do, and we are no longer responding to G-d's command when we do it. That is a fundamental problem in our relationship with the divine.
The addition of a new commandment wholesale is even worse. It indicates a lack of confidence in the Torah, in G-d's words. It demonstrates our arrogance that we think we can change (for the better) a work given to us in trust. It shows a fundamental lack of humility. Both methods of transgressing the prohibition on adding to the Torah indicate a fundamental flaw in our relationship with G-d. They show that we either aren't performing commands truly for Him or don't believe in the perfection of His word. However, it is relatively easy to maintain this prohibition. It is relatively easy not to add to the Torah. Most of us find the Torah sufficiently demanding without introducing extra challenges. In truth, this mitzvah is a challenge to us not at the level of keeping it but at the level of preserving its intent. Are we content to merely not add, or do we heed the call to give ourselves more fully to G-d's revealed word?