Answer: King David is the foundation of the whole system of correction, Malchut, and so whatever he wrote in the Book of Psalms is the basis of a person’s appeal to the upper force. All the religions use it.
Malchut is the foundation of the whole world. Ecclesiastes, on the other hand, is not about an act like King David’s book, but about thought, about study. This is not only an appeal to the Creator but an appeal to a person’s internal world. This is the reason that the Book of Psalms and Ecclesiastes are closest to man.
It seems that the Book of Psalms doesn’t turn to the Creator but to a person, although King David actually turns to the Creator.
From the Kabbalah Lesson in Russian 9/25/16