FOR NOW AND FOREVER
The portion opens with Moshe gathering all the people "That you should enter into the covenant of the L-rd, and into His oath which the L-rd G-d make with you this day." Included in this oath are not only that generation, but "with those that stand today before the L-rd our G-d, and also with those who are not here with us this day" (v. 14). Both Rashi and Ramban explain that this comes to obligate all future generations in this covenant. The Abarbanel asks "By what right did that generation, which had stood at Sinai, obligate all the following generations to keep the Torah, when the other generations were not partners to that covenant?"
Nachshoni cites many different answers to this question. The Talmud seems to indicate that the souls of all future generations were present at that time. The Torah does not mean 'those who are not with us', but those who are with us, but only in spirit, not in body. According to this explanation our original question disappears, but we have to understand how souls, which do not yet have free choice, not having entered into a physical body, are able to make a decision. The answer must be that even though once the soul enters a body the person may think that they no longer wish to keep the covenant, in reality their true essence is their soul which does want to keep it. The corollary of this is that though we may sometimes act as if we have forgotten about G-d, this is only an aberration brought about by the yetzer hara influencing the body, but in essence a person always wants to do what is correct.
Rabbeinu Bachaye explains that one generation may legally obligate later generations in the vows which the earlier generation have made. Since the earlier generation are the roots from which the children grew, anything which affects the roots, also affects the branches. The first generation gives life to later generations, and in return can obligate them with their debts. Tevye the milkman tells us the importance of 'Tradition'. However, this explanation tells us that not only do we continue with our Judaism because 'that's what we've always done', but because our very existence is dependent upon continuing in the ways of our ancestors.
The Malbim gives several explanations. There is a principle in Halacha that a person may not cause a person a debt or loss without their knowledge, however they can cause them benefit even without their knowledge or consent. The Mitzvot are to our benefit, as the Mishna states (Maccot 3; 16), "Rabbi Chananya ben Akashya said: The Holy One Blessed be He, wanted to give merit to Israel, therefore He gave them a copious Torah and many commandments, as it is said, 'It pleased the L-rd for the sake of Israel's righteousness to magnify the Torah and make it glorious'". Therefore the generation of the desert were able to obligate future generations in this covenant, because ultimately it is for their benefit. We can see from this the tremendous gift that G-d has given us through offering us the Torah, and how happy we should be in having the chance to observe the laws. This is part of the reason that G-d punishes the nation so severely for not keeping the laws with happiness: "Because you did not serve the L-rd your G-d with joyfulness, and with gladness of heart ... therefore shall you serve your enemy..." (Deuteronomy 28; 47-8).
The Malbim also explains that G-d did not need the Jews to accept the covenant upon themselves. Even had they not agreed to the terms and conditions, G-d could still have expected them to keep the Torah and its laws, since He created them and redeemed them from Egypt. The Jewish people were thus acquired by G-d, and He can instruct them to do anything that He wants. However, in His mercy G-d gave the people the chance to accept the Torah voluntarily, and thus earn further merit for themselves. When the Jews were given the choice to accept the Torah, and did so voluntarily, they strengthened their faith to such an extent that it became deeply embedded in their souls, and was passed on to all future generations. This is one meaning of the Talmudic statement (Shabbat 88a) that even though G-d forced the Jews into accepting the Torah at Mount Sinai, they voluntarily accepted it upon themselves in the days of Mordechai and Esther. They realised then the kindness that G-d had done for them in saving them from Haman, and with renewed faith they accepted the Torah unconditionally.
Furthermore, since the covenant is unilateral, G-d will cause events that will bring the Jews back to the correct path if they fail to live up to what is expected of them. This is a mixed blessing. On the one hand G-d will never allow us to forsake Him, and so will never abandon us. On the other hand, in order to bring us back to the proper behaviour G-d may need to cause us oppression and hardship, until we turn to Him in repentance, in recognition of our behaviour which caused the evil to befall us.