This week's parsha describes one of the most important changes in halachah that takes effect in Eretz Yisroel when the Beis Hamikdash will be built. It is the prohibition against sacrificing animals in any place other than the Beis Hamikdash. The Torah describes the prohibition against these sacrifices in a very peculiar way, saying (Devarim 12:8), "you shall not do there as we all do here today, each person doing that which is right [yashar] in his own eyes." Usually, the phrase "right in his eyes" is somewhat derogatory. In Tanach (Shoftim 17:6 and other places) it denotes lawlessness and anarchy. But here it refers to something which is a mitzvah in its time and in its place; it is not at all negative.
The gemara explains that 'right in his eyes' refers to a certain type of sacrifice. Before there Beis Hamikdash was built, people were indeed allowed to sacrifice on makeshift altars (bamah), and performed a mitzvah thereby, but could only do so with regards to sacrifices that were voluntary, not obligatory. Thus, the type of sacrifices that a person would offer when he sinned, and the like, could not be brought in this makeshift altar. It is only when they came to the Land of Israel, and built the Beis Hamikdash, that they could they now bring those sacrifices that are obligatory, such as a sin offering, etc.
The characterization of this period of time is therefore, "each person as he sees fit" versus a period of time when one could bring other sacrifices as well. It's very hard to understand that this should be a core definition of Klal Yisroel before the Beis Hamikdash was built, versus the state of Klal Yisroel after the Beis Hamikdash was built. To us it seems to be a mere technical detail in the specifics of the laws of sacrificial offerings.
Let us consider people's relationship with Hashem and the truths of Torah. When a person begins his journey to finding Hashem and truth, the initiative is always one's own. The paradigm of this is Avraham Avinu, who came from nowhere and on his very own came to the realization of Hashem and the truth of His Torah. It is described in seforim as "chessed" which in a very specific sense means "kindness" but in the bigger sweep of things, chessed's main emphasis is on its voluntary nature; it is motivated by one's own personal aspiration and not by any obligation. Thus, every type of spiritual undertaking must start with that type of self-motivation. When asked, "why are you doing such and such?" the answer is, "because I have found this to be true; I have come to the realization that this is right." However, when a person has already, in fact, discovered Hashem and firmly established the emess of Hashem, an important change comes into his perception of things. Hashem is now a given, an established entity, and is no longer dependent on the person's recognition thereof.
Let's draw an analogy to this. Imagine a scientist is probing the reason for a certain physical phenomenon. He has a hunch and devises various experiments to prove the veracity of that hunch. At this point it's his theory and idea. But if, after testing and observing and recording, it is found to be in fact true, it is then understood to be a fact in its own merit. It is no longer the scientist's idea, but rather a fact, with the scientist merely getting the credit for making us aware of this fact.
The same is true concerning a person's own search for the Divine and His truth. Similarly, it is true about Klal Yisroel's search for, and eventual establishment of, the Divine truth. The era of Klal Yisroel in the desert was their era of search, of wandering. There was a lack of "permanence" to their religious recognition, for Hashem travelled with them wherever they went. Coming into the Land of Israel, however, meant that this personal type of experience and search would now become a given rock-solid point of reference. This is what the Beis Hamikdash is; it is described (Devarim 12:9) as, "the inheritance and tranquility". It means that at this point we are enabled to worship Hashem not only through personal recognition, but through acknowledgement and obligation. We can now offer those sacrifices that stem from obligation, rather than voluntary donation.
In some ways this runs counter to our sense of things. We tend to feel that doing things voluntarily, out of personal recognition and offer, is the ultimate in Divine service. It certainly adds a lot to one's deeds when they are inspired, as well. But the ultimate recognition of Hashem lies in this statement that, "whether I feel like it or not, it is emess, and like it or not, I'm obligated". One donates willingly to a cause of one's own; but unwillingly paying taxes is the recognition of someone's sovereignty over me.
This, then, is what the psukim are telling us. At first it says (ibid), "do not do there (i.e. in the Land of Israel) as we are doing today; each person [bringing the sacrifices] that are right in his eyes." Once they come into the Land of Israel and build the Beis Hamikdash, the possuk says (Devarim12:28), "listen to all of the things that I'm commanding you... to do the good and the righteous in the eyes of Hashem." From this point onwards, what establishes the good and the right is not our personal sense of it, but rather Hashem's.