All of Torah was given to Moshe, who taught it to Klal Yisrael. There are however, two instances where a particular portion of Torah given to the Jewish people had the involvement of another party. One is the case of the 'mekosheish' (Bamidbar 15:32) who deserved the death penalty for desecrating Shabbos, but it was unclear which type of death penalty. The other instance was the case of the daughters of tzlafchad who sought clarification for the laws of inheritance regarding a daughter when there is no male child. In both those cases Chazal find it necessary to explain why it is that these people were the ones involved in bringing this particular teaching to Israel.
However, in this week's parsha we have something of a different magnitude, regarding people who could not bring a korbon Pessach because the Torah forbids those who are defiled from bringing it. These people did not inquire as to a mere clarification of the existing law, rather they demanded that they be given a new opportunity to fulfill this mitzvah. This brought into being a whole new mitzvah that had not previously been revealed at all; it was as if they had indeed succeeded in creating a new mitzvah. This is an extraordinary phenomenon, and some understanding of how and why this happened is needed.
Let us consider for a moment two different attitudes towards mitzvos. One observant Jew takes the Torah to be God's will and dictum and feels obliged to fulfill it. He fulfills it to the best of his abilities and to the full demands of the Torah, but he sees no reason to demand or want more of the Torah. He approaches mitzvos like all duties that we fulfill; one's job is to fulfill one's duty appropriately and all obligations ends with that. It seems that we have described a truly virtuous Jew.
However, in this parsha we have a sign of how sorely lacking the attitude described above is. Rashi quotes Chazal describing Klal Yisroel's departure from Sinai as a negative event. It is unclear why, but in a few places in the Midrash, and as quoted by rishonim, it states that it is, "because when Israel left Sinai, they ran away like a child running away from school". This cannot mean that they left Sinai before they were supposed to, for their movements and journey were very specifically coordinated by Hashem himself. When the divine cloud folded and began moving, that is when Israel began moving - not a minute before, and not a minute after. So what does it mean that they "ran away"? It means that they left exactly at the moment they were supposed to, but with a sense of relief that it's over. [Some versions of that above Chazal describe their departure as being, "like a child who leaves school and runs off.] This is an attitude towards Torah that expresses that Torah and its obligations are merely just that - obligations. Obligation that is, indeed, fulfilled meticulously, but nothing more than that.
One is not faulted for not demanding more obligation so long as one has fulfilled the obligation to perfection. But Torah is a lot more than obligation; it is opportunity as well. A person needs to see his sense of fulfillment and sense of self expressed in Torah. Every mitzvah performed, every part of Torah understood, builds the person bigger and better. This is in line with the words used by the people who were complaining about not being able to bring a korbon Pesach. They used the phrase, "lomo nigari - why are we losing out?". They did not see the mitzvah as a mere obligation, from which they had been duly exempted. Rather, they understood each and every mitzvah to be an additional brick in building a person b'tzalomo k'dmuso. Therefore, they came with a demand, and it was the demand itself that caused Hashem to give them a so-called "new" mitzvah. This could not have been given by Moshe, because then it would have simply been part and parcel of all other obligations. This mitzvah represented the quest for fulfillment that is the inner basis for mitzvos.
This is something that unfortunately becomes somewhat lost to us, when we speak about "Torah observant Jewry", when we speak of Orthodoxy. We toe the line, we follow the regulations, and we dot every "i" and cross every "t". Certainly, that is the beginning of Torah observance, for if a person takes Torah and mitzvos to be only spiritual fulfillment, and not obligation, he is missing the basis for it all. It is like someone who is law-abiding because he feels like it, which means in effect that he does not recognize the authority and the validity of the government. That is something that is negative.
We have one more example in this week's parshah, of someone who saw mitzvos as fulfillment in addition to obligation. It says that Aaron was upset that he did not participate in the consecration of the mishkan. He had done no wrong, for he was not commanded to bring any sacrifice; all the necessary sacrifices were duly brought by the nesiim. But Aaron saw mitzvos as opportunities rather than obligation only.
And that is why Hashem told him, "You will light the menorah as your act of dedication of the mishkan." Hashem then added, "Yours is greater than theirs, for yours will last eternally". The reason for that difference is that when a person is occupied with fulfilling obligations, it comes to an end at some point; when the obligation has been duly fulfilled, it is over. Not so the person who yearns and seeks mitzvos, recognizing that they are building him and bringing out the best in him. For a person like that, the chain of mitzvos and opportunities is indeed eternal.