Our Torah reading gives the commandment for the Jewish nation to recite the blessings and curses on Mount Gerizim and Mount Eival, after they have entered the Land of Israel. The Mishna (Sota 7; 5) describes how this took place: "Once the Jews had crossed the Jordan river and come to Mount Gerizim and Mount Eival which are in the Shomron; Six of the tribes ascended Mount Gerizim, and six ascended Mount Eival, while the Cohanim, Levi'im and the ark remained in between at the bottom. They turned towards Mount Gerizim, and began with the blessing, 'Blessed is the person who shall not make a graven or molten image' and all the people answered 'Amen'. Then they turned toward Mount Eival, and recited the curse 'Cursed is the man who will make a graven or molten image' (Deuteronomy 27; 15), and all the people answered 'Amen'. This continued until they had completed the blessings and the curses."
The problem with this Mishna is the fact that the Torah doesn't actually list the blessings that are mentioned. In our Torah reading we only have the curses. We have to rely on the principle (Nedarim 11a) that from a negative we can infer the positive to work out that we should recite the blessings also.
The Torah always tries to use positive language. Why in this case did G-d only give us the curses? Would it not have been better to list the blessings and leave us to deduce the curses on our own?
We know that there is nothing which remains stationary, either physically or spiritually. Anything which is not moving upwards is decaying downward. This is explicit in several places in the Mishna, e.g. "Hillel said ...He who does not increase his knowledge decreases it" (Chapters of the Fathers 1; 13). Using this concept we can explain why the Torah only listed the curses. Had we been given only the blessings, we might have thought that keeping the Torah led to blessing, but not keeping the Torah had no consequences. However, since the Torah lists the destructive results of transgression, we can infer the blessings that come into the world through observance. The Torah is teaching us that there is no such thing as taking 'time out', and doing actions which have no consequence whatsoever. Every act that we do either brings blessing into the world, or the opposite, there is no such thing as a 'pareve' action.
The Kli Yakar gives another two explanations. Firstly he explains (verse 12) that the main blessing which is promised will only occur in the World-to-Come, which is hidden from us now. As a sign that the real reward is not in this world, G-d chose to hide the formulation of the blessings, so that they are not explicit in the Torah. The reason that the reward must be in the World-to-Come is in keeping with the Talmudic maxim that the reward from G-d is always far greater than the punishment. Though G-d will "visit the sins of the fathers on the sons and grandsons for four generations" (Exodus 33; 7), the reward is for one thousand generations (Deuteronomy 7; 9). The consequences of Mitzvot are thus 250 times more powerful than the results of sin. Thus, though a sin could be punished in this world, reward for Mitzvot occurs only in a world which is infinite. Since it has no physical constraints there is no limit to the effects of the Mitzvah.
Alternatively the Torah tells us the curses, because we know that through the repentance process sins and curses can be turned into merits and blessings. "Reish Lakish said, 'how great is repentance, for through repentance premeditated sins are accounted as merits, as it is said (Ezekiel 33; 19) "When a wicked man returns form his wickedness and practices justice and charity, he shall live because of them"" (Yoma 86a). Therefore the Torah gives us the curses, not only to infer the blessings from them, but to teach us that through repentance the blessings can actually be contained within the curses.