We are exhorted not to add to the Mitzvohs of Hashem, nor should we detract from them. Examples of adding are wearing Tzitzis of a five cornered garment, or sitting in the Succah for eight days instead of seven. An example of subtracting would be having three sections in one’s Tefillin when in reality four are required.
The problem with detracting from the Mitzvohs is elementary for the person is failing to fulfill his requirements. What could be wrong with adding to the Torah, one is doing over and above his required obligation, what is problematic with that?
My father taught me in the name of the Dubno Maggid that if a person can add he will assume responsibility and there is the possibility that he will end up subtracting. The Maggid brought out this point with a beautiful parable:
A person who was destitute went to his wealthy neighbor and asked to borrow a spoon. The rich person was reluctant to lend it out but the poor man prevailed. The next day, he returned the spoon he had borrowed together with another small spoon.
The rich man was taken aback “Why are you giving me two spoons? I only loaned you one spoon.”
The poor man replied “Last night the spoon which you had loaned me gave birth to a baby spoon, in truth both spoons are yours so I returned two spoons.”
The rich neighbor was still a little stymied but he nonetheless accepted the two spoons.
Several days later, the poor man returned and asked to borrow some silver candlesticks. The rich man was a little nervous but decided to take the plunge as after all the poor man did return the spoon.
The next day the poor man returned the silver candlesticks with matching miniatures, “What is this?” spluttered the rich man. “Twins” answered the poor man, “last night your candlesticks gave birth to twins, Mazal Tov Mazal Tov”. The rich man was now pleased with his successful lending.
Several days later, the poor man came to borrow an entire set of silver cutlery. By now the rich man had discarded any traces of reluctance, brimming with excitement he gave the poor man the entire set of flatware, wondering how many babies his silver will bear.
The next day the poor man did not show up, the rich man was a little concerned. After a few more days when he saw that his silverware had not been returned, he asked his friend “Where is my silverware?”
“I am sorry” replied the poor man, “but your silverware passed away.”
“Passed away?” screamed the rich man,”who has ever heard of silverware passing away?”
“Well” responded the poor man, “who has ever heard of a spoon that gave birth? Yet when I gave you two spoons, you took them without saying a word. Now if something can give birth it can die”.
The same thing applies to mitzvos if people can add they will come to subtract. In this analysis the prohibition against adding is none other than a safeguard to prevent him from subtracting.
I think it is possible to suggest a different approach. The wording of the Torah is “You should not add to the matter which I command you, nor should you subtract” that extra phrasing in the middle – the matter which I command you – is the key. We do Mitzvohs because Hashem commanded them, even logical Mitzvohs such as honoring parents are performed because it is the Divine will, not because we think that it makes sense. The Rambam writes that a non-Jew who wants to become a Ger Toshov has to accept the Mitzvohs because Hashem relayed them to Moshe on Mt. Sinai. Even though the Noahides laws are logical, do not kill, do not steal etc… He has to accept them not because they are intellectually sound but because Hashem commanded them. The person who adds to the Mitzvohs demonstrates the assertion of his discretion in his mitzvah performance. The character of his actions is authorized by his puny mind instead of handing control over to Hashem. Of course we are meant to apply our intelligence in vague areas or try and realize what Hashem wishes in this situation, but the general attitude has to be one of fulfilling the divine will.
Why do you help an old lady cross the road?
One may not shine silver on Shabbos even without using silver polish (See Shulchan Aruch 323:9)