"Your life will hang in the balance, and you will be frightened night and day, and you will not be sure of your livelihood" (Devarim 28:66).
Rashi explains that in exile the Jewish People will never be safe from violence inflicted by others. Their livelihood will be based on daily purchases and they will never be assured that the markets will be available to them. In Menachot (103b), Chazal explain that this Pasuk refers to a situation where one has very little to eat and must find food daily. This situation is characteristic of unhappiness: not knowing where one's food will come from and uncertainty of what tomorrow will bring, puts one in an unhappy mood.
We must attempt to understand this curse describing the depths of depression in light of the following statement of Chazal:
The students of Rabbi Shimon bar Yochai asked him, "Why didn't the Manna fall once a year, enabling the people to gather a full year's supply at one time?" Rabbi Shimon replied with a parable. "A king had one son and provided him with food once a year. Consequently, the son only visited the father once a year, as he only needed his father at that time. As a result, the king began providing his son with food on a daily basis, so that he see his only son every day. So too, a father of each family of Bnai Yisrael would be concerned daily that perhaps there would be no Manna for his family. Consequently, all of Klal Yisrael would turn to Hashem in prayer, entreating Him for their daily sustenance."
According to Chazal, the need to turn to Hashem for one's bread on a daily basis is not a curse, but a blessing. This situation strengthens the bond between Hashem and Bnai Yisrael through their constant communication. Obviously, the generation that received the Torah on Har Sinai was not subject to the harsh curses which are specified in this Parsha.
Rav Chaim Shmuelevitz makes a profound observation in responding to this question. The Torah does not mention a lack of food as a likely curse. It merely states that the fear and uncertainty regarding our daily nutrition is overpowering. The fear of lack of food and the inability to feed one's family will be intolerable for men who have little or no faith in Hashem. An individual who does not learn to rely on Hashem as the only Source of blessing can be overwhelmed with uncertainty and unwillingness in his daily wait for bread. The suspense of "What will tomorrow bring?" is terrifying for people with little faith.
People who trust Hashem, on the other hand, view this as a source of blessing, for it produces a closer relationship with Hashem. Suddenly, daily bread takes on a new image: it becomes associated with the essence of manna. The ability to trust in Hashem and not to worry about tomorrow can transform the harshest of curses into blessings.