Establishing the King

Rashi: "He was" i.e. G‑d

"was King in Jeshurun"
i.e. the yoke of G‑d’s sovereignty was always upon them.

"whenever the sum total of the people were gathered"
At every gathering of "heads"-- whenever their number [i.e. "head-count"] is gathered. These people are worthy that I should bless them. Another explanation: When Israel is gathered together in a unified group, and there is peace among them, God is their King, but not when there is strife among them.

Baal HaTurim: The children of Israel established Him — G‑d — as their King, meaning they accepted Him as their King. Alternatively, G‑d established Moses as king over Israel.

Targum Yonatan: The sons of Israel said, Moses commanded us the law; he gave it for an inheritance and possession to the congregation of the house of Jacob. And a king shall arise from the house of Jacob, when the heads of the people are gathered together; unto Him shall the tribes of Israel be obedient.

Ramban: This alludes to the name of G‑d, Who was King over Israel when they were upright and all the heads of the people and all tribes gathered together. This verse too is connected with the Revelation at Sinai, in the sense that Israel will always say, "We will never alter it or change it." He mentioned the Torah in general and the Kingdom of Heaven in particular, for whomever acknowledges the Kingdom of Heaven denies idols and acknowledges the entire Torah....when Israel sit in council on earth, His Great Name is praised on high.

Ohr HaChayim: Our verse hints that the Kingdom of Israel will endure only if the heads of the people assemble together to keep the peace themselves, not if the leaders are split. For if they are, the tribes won't be able to maintain their unity. A look at our history shows that as long as people were united under David and Solomon, the nation was extremely successful, whereas as soon as a split developed, the fortunes of the Jewish people took a turn for the worse.

Tzeror HaMor: Moses explains that seeing that the Torah has many warnings and prospective penalties for not observing its commandments, it would have been understandable if some of the people had readily embraced Torah whereas others like Esau would have rejected it. It is to the everlasting credit of the Jewish people that all of them without exception accepted it enthusiastically, so that G‑d and His Torah became King in Israel.

Mei HaShiloach: A state needs a king so the citizens don't break the laws. Yet, if all would behave, and act truly and honestly, there would no longer be a need for a earthly king, but a Supernal King would still be important. In this sense we say in our High Holiday prayers: "You, G‑d our Elokim, will soon reign over all Your works... for You shall remove the governance of iniquity from the land." His throne will be magnified, not like that of an earthly king.

The Lubavitcher Rebbe: This verse speaks of Jewish unity, a level which is in fact higher than Jewish brotherly love. For even when you fulfill the command to "love your fellow like you love yourself," there still remain 2 separate entities: your fellow and yourself. But when unity is achieved, all the Jewish people come together as one single entity: "the tribes of Israel together." Still, the Torah did not mandate an explicit command that the Jewish people reach the state of unity, because unity is a direct consequence and an inevitable progression from loving one's fellow as one loves oneself.

(The Arizal in his discussion of Iggulim/circles and Yashar/lines, notes that Iggulim hints that each Israel soul stands directed before the blessed G‑d, and no one has an elevated status over the other. "In the future, G‑d will make a circle dance for the righteous". That is where each one is equally close. Yashar however implies different levels of elevation. In the future both Yashar and Iggulim will be together as one. The sanctity inherited by the Patriarchs is called Iggul, and the holiness one earns by means of his actions will gain himself spiritual elevation. Then he too will be recognized as the one whom the Blessed G‑d loves, and even within the circle, will be seen as closer than the rest. This is "a king in Jeshurun," who is Moses, and his greatness will be recognized.)

Zohar Acharei Mot 59:
Another explanation for "Behold, how good and how pleasant..." These are the friends, when they sit together and don't sit apart from one another. At first they appear as people at war with each other, wanting to kill one another. Afterwards, they return to each other in brotherly love. What does the Holy One, blessed be He, say? "Behold how good and pleasant it is for brothers to dwell together (gam, 'also') in unity." The word gam comes to include the Shechinah. Moreover, G‑d pays attention to what they say, and has enjoyment and rejoices with them. This is the essence of the verse, "Then they who feared G‑d spoke to one another: and G‑d hearkened, and heard it, and a book of remembrance was written before Him..." (Malachi 3:16)
(Rabbi Shimon now turns to his students who are near to him and he says to them:) My friends, just as you have had affection for each other until now, do not part from one another from now on until the Holy One, blessed be He, rejoices with you. He will call peace upon you and, for your merits, peace will prevail in the world. This is the meaning of the verse, "For my brethren and friends' sakes, I will now say, Peace be within you." (Ps. 122:8)