“The blessing - that you shall heed [the mitzvos]... and the curse - if you shall not heed [the mitzvos] and you stray from the path."
When mentioning the blessing, the Torah tells us that you shall follow the commandments of Hashem, but the curse comes about when not only are the mitzvos not performed, but also the sinner strays from the path laid out for him. If one is not performing the mitzvos, how much more can he stray from the path?
Simply put, one may agree that he should follow the mitzvos, but he does not for whatever reason. These may be acts that people excuse such as being dishonest in business, talking instead of davening, or ignoring the laws and customs of family purity. While he is not careful in these things, he understands it is wrong. Were he to deny these mitzvos altogether, that would be a more grievous sin.
However, we can explain this disparity with a basic understanding given by the Maharal. We know that during the giving of the Aseres HaDibros, the Ten Commandments, Hashem said Zachor and Shamor at the same time. The obligation to remember the Shabbos and to guard it were mentioned at precisely the same instant. The Maharal explains that this was done specifically by Shabbos, though it applies in other cases, because Shabbos is the most striking example of the following concept:
Remembering Shabbos refers to the mitzvos asei, the positive commandments, of the day, such as Kiddush and Oneg Shabbos – delighting in it, while “Guarding Shabbos” refers to the negative commandments, such as the prohibition against lighting fires or discussing business matters.
In order to successfully perform and enjoy the positive commandments, one must use the prohibitions as a framework for keeping Shabbos. If one does that, and is careful not to transgress, then he will be able to perform the positive commandments with joy, enjoying the spiritual aspect of the Shabbos. If, however, the negative commandments are merely barriers for him, obstacles that he resents, then he will never truly enjoy the Shabbos and all its delights.
The same can be applied to all the mitzvos. If one performs the mitzvos with joy, then that in itself is a blessing. He has a life full of meaning and fulfillment. However, if he goes astray, and does not utilize the mitzvos as a guidebook, then not only will he not be able to perform the mitzvos properly, but he will suffer the curse of an empty life. The mitzvos are meant to bring joy but one can only realize this joy when he builds his life around the Torah and understands that it is his instruction manual for living.
