The reconciliation between Joseph and his brothers is the foundation and serves as the model for the future reconciliation between the tribes of Israel in the north and the tribe of Judah in the south. This link is described in the Haftarah for this week, discussing the "wood" of Judah and the "wood" of Ephraim, as a political merger. "And I will form them into one nation in the land, in the mountains of Israel, and they will all have one king. They will no longer be two nations and never again will they be divided into two kingdoms" [Ezekiel 37:22]. At first the merger is only in a secular and technical way. But the experience of unity brings the nation back to its true identity, and the people will rid themselves of their idols: "And they will no longer be contaminated with their idols and their abominations and their sins and I will rescue them from their dwellings," and as a result, "they will be My nation and I will be their G-d." [37:23].
Only after some measure of return to the original identity of the nation does a leader appear who has traits similar to those of David. "And My slave David will be their king, and there will be one shepherd for all" [37:24]. Here David is called a king, but in the next verse it is written, "And my slave David will be their leader (nassi) forever" [37:25]. The difference between a king and a nassi is that the government of a king has inherent authority, with the power extending down from above, while a nassi draws his power from the nation that appointed him.
A central and powerful government is necessary when society is morally weak, but when the society is morally mature there is no longer a need for a king, and it is sufficient to have a nassi. This leader, the Nassi, must be descended from the House of David, because this family carried the burden of the hopes of the nation in the days when it was downtrodden. Therefore, without this dynasty we would be lacking the symbolic high point of our national aspirations. (See "Writings from His Holy Hand," Rav Kook, pages 36-37, about the dispute between Rav Yosef and Rabbi Hillel.)
The transformation from a king to a nassi comes as a result of adopting two moral values: the Torah and the Land. Appointing a king brings the nation back to observing the dictates of the Torah: "They will follow My laws, and they will obey My decrees." [37:24]. Performance of the mitzvot is in reply to external coercion, which corresponds to the nature of a royal government, as is written by the Rambam – that the Messiah, the King, will force the people to observe the Torah (Hilchot Melachim 10:2). But during the process of becoming mature and renewing our national identity, we come into contact with nature itself, through our contact with the land: "And they will dwell in the land which I gave to my slave Jacob, where your ancestors dwelt, and they will live there, they and their children and their children's children, for all eternity" [37:25]. The Land of Israel, where secular life is also holy, renews the healthy natural feelings of people. The return to a normal state for a number of generations, to a life that is full in every sense of the word, will lift up the soul of the society to a state where coercion is no longer necessary – then "my slave David will be a Nassi for them forever" [37:25].