Moshe lectures his people to obey Hashem’s commandments, because they are for their benefit:

All the commandment that I command you today shall you observe to do, so that you may live and multiply, and come and possess the land that Hashem swore to your forefathers (Devarim 8:1)

Of course, Hashem has demonstrated through His Providence that He takes care of the people of Israel:

You will recall all the way/road on which Hashem, your G-d, has led you these forty years in the wilderness…(8:2).

But Hashem’s purpose was not only to provide material sustenance; He wanted to teach them to rely on Him. His aim was to take former slaves – people, entirely dependent on their human masters  and transform them into ennobled servants of Hashem. At times, Hashem’s method of education may have seemed harsh:

…in order to afflict you, to test you, to know what is in your heart, whether you will keep His commandments or not. And He deprived you, and starved you, then He fed you the manna, which neither you nor your ancestors knew; in order to make you know that it is not by bread alone that man lives: rather, by all that comes forth from the mouth of Hashem does man live. Your garment did not wear out on you, nor did your foot swell, these forty years. And know in your heart, that just as a man disciplines his son, so does Hashem, your G-d, discipline you (verses 2-5).

This is “tough love”, which trains the people to appreciate all that Hashem does and to be loyal to Him as a consequence:

You shall keep the commandments of Hashem, your G-d, to walk in His ways and to revere Him (verse 6).

The opposite of slavish dependence on man, as occurred in Egypt, is not independence, but rather a healthy reliance upon Hashem. It is this reliance that the people will need to cultivate in the Land of Israel:

Because Hashem, your G-d, is bringing you to a good land, a land of water brooks, springs and depths, that emerge both in the valley and in the mountain; a land of wheat and barley, of vine, and fig, and pomegranate; a land of oil-olives and honey; a land in which you will eat bread not in parsimony – you will lack nothing in it  a land whose very rocks are iron, and from whose mountains you can quarry copper (verses 7-9).

This land is bountiful, but the people must work it in order for it to produce, for which they need to be beholden to Hashem.

At this point, Moshe makes a statement which can be understood in two different ways:

And you will eat and you will be satisfied, and you will bless Hashem, your G-d, for the good land that He has given you (verse 10).

Ramban understands this as the culmination of all that has preceded:

“When you remember the slavery of Egypt and the deprivation of the wilderness, and when you will then eat and be satiated in the good land, you will bless Hashem for it.”

This verse is, according to Ramban, mainly a prediction of future events. The blessing which you will then utter will come to your lips naturally, if you but remember your past history with Hashem.

Ramban is well aware that this verse is the source for the mitzvah of Grace after Meals (Birkat Ha-Mazon), which would translate the verse thus:

“And when you eat and are satisfied, you shall [i.e., you are commanded to] bless…”

However, he insists that this reading is secondary.

Sforno, on the other hand, holds that the verse is primarily a commandment:
“And you shall bless Hashem your G-d  So that you will remember that it is from Him that you have these things.”

In a sense, Sforno reads this verse not as the conclusion of verses 1-9, but as the reason for the verses that follow:

Beware lest you forget Hashem, your G-d, by not keeping His commandments, His judgments, and His statutes, which I commanded you today; lest you eat and are satisfied, and you build good houses and settle down, and your cattle and flocks multiply, and silver and gold are multiplied for you, and all that you have will increase. And then your heart will become haughty and you will forget Hashem, your G-d… (verses 11-14)

In order to prevent this presumptuousness, the Torah commands us to acknowledge our debt to Hashem’s goodness by reciting Birkat HaMazon.

Ramban and Sforno, it seems, have two different views of human nature. Ramban maintains that as long as we are aware of what we have received at Hashem’s hand, we will naturally express our gratitude. Sforno feels it is natural for our own accomplishments to “go to our heads,” leading us to forget our dependence on Hashem:

And you will say in your heart: “My Strength and the might of my hand have made me all this wealth” (verse 17).
Therefore, the Torah commands us to recite the Birkat HaMazon.

As explained in Berachot (21a, 35a, 48b), Birkat HaMazon must include thanks for our nourishment and for the land of Israel, including Jerusalem and the Temple. This will prevent both ingratitude and complacency.

Insofar as human nature is concerned, both Ramban and Sforno are right. We can become overly self-reliant, and we can become forgetful. But Birkat HaMazon, when recited with thoughtfulness and sincerity, will serve as a reminder of what we already know  that Hashem gives us our very lives. With this in mind, our natural goodness will express itself in gratitude to Him.