The source for saying Birkat HaMazon may not be as obvious as we normally assume. Our likely assumption comes from being accustomed to reading the first three words from Devarim 8:10, “Ve’achalta, vesava’ata ou’verachta” (you shall eat, and be satiated and bless),” in insolation — as we find them in Birkat HaMazon itself. Read that way, it seems pretty clear that the Torah is telling us that when we eat properly, we should bless G-d. Significantly, this conclusion is not just liturgical, but it underlies the halacha as well.
Taken in context, however, it is less clear. Ramban is aware of this and tells us not to read the second part of the verse in its most obvious way — as the rest of the verse tells us that the blessing that we are supposed to give to G-d after we have eaten, is specifically for the land He has given us. Ramban however tells us to read it as if there was a conjunction between our first three words and the rest of the verse. Hence, we should bless G-d more generally and bless Him for the land, as well. But not only is that not the simplest reading of the verse, it seems to take the verse out of its context, which is all about the great land that G-d is giving the Jews.
On the one hand, the notion that the halachic understanding of the verse can disagree with its simple reading has always been a controversial point — especially when we can be sure that the rabbis meant their understanding to be a bona fide interpretation of the text and not just what we call an asmachta be’alma (a memory device). On the other hand, Jews have traditionally been comfortable with the idea that there is more than one valid way to read the text — as the Divine words invariably contain much more content than can be subsumed under just one reading.
In the case of the verse in question, I would not only suggest that the halakhic reading of the verse is a valid — if less obvious — one. I would go further and suggest that this reading is finding a way to make the descriptive (what is) into the prescriptive (what should be), which is really what halacha is about, more generally. I will explain:
Shadal has already pointed out that the simple reading is speaking about blessing G-d as a natural response to the bounty that the Jew will encounter in the Land of Israel. Any slightly religious individual will be familiar with this response to overwhelming bounty. When we receive more than we could ever imagine, it is natural to want to thank G-d. While this is a good and healthy response, it can hardly be a major pillar of religious life. For how often is the average person overwhelmed in this fashion?
In fact, Shadal points out that the verses that follow this speak to the more common situation that will happen over time, as the Jews become more used to the bounty of their land. Though the land will continue to serve them well, they will be likely to forget G-d’s involvement with it altogether. And what of those that (like almost all of us today) are not connected to the land at all? While any bounty gives us cause for thanks, we are more able to see G-d’s hand in the production of food from the clearly natural world that He created. If one accustomed to the land’s bounty is likely to forget his gratitude, someone off the land is even more likely to do so. That is what is.
But the rabbis understood that the highly appropriate response that the verse speaks about reflects a latent appreciation that is with us more commonly. When reminded that my food or my paycheck comes from G-d, I am also appreciative. The only difference is that I need the reminder.
But the feeling of gratitude when I bless naturally and when prompted by the reminder is ultimately the same. Accordingly, the rabbis understood that once natural gratitude is established by this verse, it reflects an attitude even more than it does an action. That is to say that the verse shows us that man is naturally grateful and knows how to express that gratitude on his own. At that point, it becomes clear to not only read the verse for the expression of gratitude that always does happen, but also for the latent expression that always can happen. As both are ultimately the same expression of the same attitude.
Hence the rabbis knew that the possibility of going from that which people do to that which they should do is all part of that which the verse communicates to us. And through such a halachic reading, they were able to translate an occasional positive action into the pillar of faith that the constant expression of gratitude to G-d — when done properly and sincerely — can actually become.