A Jew is commanded to recite one hundred blessings each and every day (Menachot 43b). We need constant reminders to ensure that we recognize the blessings of G-d and to remind ourselves that in all of our actions we are to reflect the Divine image. While most of the brachot we make consist of man acknowledging G-d as the master of the world, the priestly blessings are an exception to this pattern. In this particular blessing the Torah commands man to bless his fellow man. While G-d is the source of the blessing He demands that we use man as a conduit for these blessings.
This partnership of G-d and man in bestowing the blessings of G-d is not by chance. While all that befalls us emanates from G-d—"one is obligated to bless [G-d] for the bad just as he blesses for the good" (Brachot 54a)—one must also acknowledge the role of our fellow man in our blessings.
Our parents, teachers, friends, colleagues and competitors are what make blessings possible. One may not say that since G-d is the source of everything that I will pay homage to Him alone. Nor can we think that if G-d wants to bestow His blessings upon me, He could have done so with or without the help of this individual. That may be true but it is irrelevant. People too are the source of our blessings and due recognition must be given to them.
Similarly, Judaism rejects the notion that since we were destined to be slaves in the land of Egypt we should not punish the Egyptian people. We dare not say that they were just agents of the Almighty, fulfilling the promise that G-d had made to Abraham hundreds of years earlier that we would be oppressed in a foreign land. By allowing themselves to inflict suffering they must necessarily suffer the consequences. A person may be the conduit for G-d's prophecy but it is man alone who is responsible for his actions.
Equally important, however, is the second part of the verse, "and guard you". All too often we have turned our blessings into curses. If we are not careful, our wealth can lead to arrogance, to disdain for others. How many families have been torn apart trying to divide the assets of a parent's estate? Wealth has the capacity to change a person, and not always for the better. The Netziv notes that these dangers are not limited to our material blessings. Scholars, including the greatest of Torah scholars, are prone to ivory-tower arrogance. They may look down on the masses, thinking they have little to learn from them.