Deuteronomy 17:14

When you enter the land which the Lord your God giveth you, and possess, and dwell in it, and you say, Let us appoint a king over us, like all the nations about me,

AND THOU SHALT SAY: ‘I WILL SET A KING OVER ME.’ In the opinion of our Rabbis [who say that “The Israelites were obliged to fulfill three commandments upon their entry into the Land: to appoint a king etc.”] this verse is like “and thou shalt surely say ‘I will set a king over me.’” This is a positive commandment, for He has obligated us to say so after conquering and settling [in the Land]. The expression is similar to and thou shalt make a parapet for thy roof [which is also a matter of obligation and not one of choice], and other verses besides these. He mentioned and thou shalt say because it is commanded that the people come before the priests of the tribe of Levi, and to the judge and say to them, “It is our wish that we set a king over us.”
It is my further opinion that this is also one of his [Moses’] allusions to future events, for so it happened when the people asked for Saul, saying to Samuel, Now make us a king to judge us like all nations, and similarly it is written there, that we also may be like all the nations, and that our king may judge us etc., Verse 20. For what reason is there that the Torah should say in connection with a commandment [‘I will set a king over me] like all the nations that are round about me’ when it is not proper for Israel to learn their ways. and neither be envious against the workers of evil. But this is an allusion to what will be, and therefore the section is written in an intermediate tense [and not in the form of a command] as I have already explained.

אשימה עלי מלך ככל הגויים, a monarch whose authority will be transferred from him to his biological heir after his death throughout the generations. This is quite different from judges, each of whom is appointed for his lifetime, with his heirs having absolutely no claim to succeed their father. The people were commanded to appoint such a hereditary position of king after they had conquered and settled in the land of Israel, the principal reason being to ensure that the Jewish people would not be like a flock without a shepherd, (Moses’ words in Numbers 27,17.) The fact is that G’d disapproved of a hereditary type of monarchy such as is customary among the gentile nations, so that He stipulated that if the people insisted on appointing a king who would start a dynasty, the initial king had to be approved not only by the people but by G’d’s representative on G’d’s behalf, as we know from Samuel I 8,18. The restrictions in appointing the original king were designed to ensure that such a king could not lead the people away from G’d’s Torah; on the contrary, they are meant for the people to see in him a shining example of Torah-observance, which in turn would inspire their own piety. Seeing that when the people asked Samuel for a king their purpose was far from what the Torah had legislated having a king for, the people were suitably punished as per the verse in Samuel I 8,18 that we referred to earlier. The prophet Hoseah 13,11 describes G’d granting the people’s wish as stemming from the fact that He was angry at them. Summing up, we may view the permission to appoint a king as being in the same category as the permission for a soldier to marry a physically attractive prisoner of war. G’d, the master psychologist, knows that sometimes in order to become wise enough to appreciate the Torah’s prohibitions, an individual, or even a whole nation, must find this out by having chosen in their own wisdom to ignore the Torah’s preferences. David’s experiences with his son Avshalom, born to such a prisoner of war whom he married because he was initially physically attracted to her, bears this out. In fact, his experience with his son Adoniah was not much better. (compare Tanchuma Ki Teytze, section 1).