The Kohanim are instructed to bless the people. KO TEVARACHO ET BNEI ISREAL, “thus you shall bless the Children of Israel.” (Num. 6,23) The words that the Kohanim are to recite are then presented. YEVARECHECHA HASHEM, “May Hashem bless you.” This seems to be a contradiction. First they are told to bless the people and then what they say is that Hashem should bless.
The truth is that the Kohen does not really know what each person wants as a blessing. He may bless someone to have a nice car and he would really want to have a nice home. The Kohen might think the congregation would like to have a beautiful synagogue when in truth they would like to have a good functioning Yeshiva. Furthermore, even if it were possible for the Kohen to ask each individual what he wants, what he asks for may not be good for him and could possibly be harmful.
Only Hashem knows what each individual wants and needs or what will benefit Klal Israel. That is why the Kohen merely invokes the blessings of Hashem upon the people. By saying that Hashem should bless you, they are really saying that they should be granted their needs by Hashem as He knows what they want and what is good for them and what is not.