Deuteronomy 14:1

As beloved children before the Lord your God, you shall not make lacerations in your flesh, nor make bare the crown of the hair over your foreheads on account of the soul of the dead.

cut your flesh ([Hebrew: titgodәdu] from “upon all the hands shall be gashes [Hebrew: gәdudot]” [Jeremiah 48: 37]) for the same reason as “any incision for the soul” [Leviticus 19: 28].

nor make any baldness as the other nations do, to this very day.

Children Once you understand that you are children of God , and He loves you more than a father loves his child, then you will not cut your flesh over anything that He does, for everything that he does is good. Even if you do not understand it — as small children do not not understand their father’s actions, yet they trust him — you should yourselves act likewise, because you are a consecrated people and you are not like the other Canaanites. Accordingly you should not act as they act. The reason for the juxtaposition of this passage with the passage you must not eat any abominable thing is: You are to be a holy nation, in your hearts and in your mouths. You must thus be distinguishable from the other nations, so that anyone who sees you will recognize you. Accordingly you must not make yourselves bald over the dead [cf.: 1], nor may you eat just any animal — for, if you are to be a holy nation, it is not fitting that you should eat that which is defiled, and thereby defile your souls.

YE ARE THE CHILDREN OF THE ETERNAL YOUR G-D: YE SHALL NOT CUT YOURSELVES, NOR MAKE ANY BALDNESS BETWEEN YOUR EYES FOR THE DEAD. This also is an explanatory commandment of what He said in the Torah with reference to the priests, They shall not make baldness upon their head etc., nor make any cuttings in their flesh, and now he [Moses] will explain that it is not merely because of the distinction of the priests which He mentioned there, They shall be holy unto their G-d,, Verse 6. that they were thus commanded. Rather, all the congregation are holy, every one of them, and all of you are the children of the Eternal your G-d like the priests; if so, you too, take heed to yourselves regarding this commandment like them. This explanation is in accordance with the opinion of our Rabbis that both verses [the one here and the one in Leviticus] relate only to the practice [of cutting and making baldness as signs of mourning] for the dead, [although “the dead” is not mentioned in the priestly injunction in Leviticus]. But it is possible that the first commandment [in Leviticus was stated] in relation to priests to state that if the priest had his hair torn out and his flesh cut, he was not fit for the Divine Service, just as He said, and they shall not profane the Name of their G-d,, Verse 6. and their Service is thus profaned [i.e., invalidated]. Here he explained that the commandment applies also to the Israelites, both prohibitions being necessary.
And Rashi wrote: “Because you are the children of the Eternal it is becoming for you to be comely and not be cut about and bald [due to the tearing out of hair].” But it is not correct, for if so this commandment should be binding even [if the deeds were done] not for the dead! And Rabbi Abraham ibn Ezra commented: “Once you realize that you are the children of the Eternal and that He loves you more than a father loves his child, you should not cut yourselves [in bereavement] for anything that He does, because whatever He has done is for your good although you may not understand it, just as little children do not understand their father, yet rely on him. For thou art a holy people, and you are not like all the other nations; therefore you shall not do as they do.”
In my opinion the purport of the expression for thou art a holy people [is to state] an assurance of the eternal existence of the souls before Him, blessed be He. The verse declares: “Since you are a holy people and the treasure of G-d — neither doth G-d respect any person, but He deviseth means that he that is banished be not an outcast from Him — therefore it is improper for you to make incisions in your flesh and tear your hair for the dead even if he perisheth in youth. Scripture, however, did not prohibit weeping for the dead since it is natural to cry when parting from beloved ones, and when they go on a journey even in life. From this verse, there is support for our Rabbis in prohibiting excessive mourning for the dead.

לא תתגודדו, as described in Kings I 18,28 ויתגודדו כמשפטם בחרבות וברמחים, “they gashed themselves with swords and daggers.” The word is also used for cutting down trees (Sanhedrin 110)

בנים אתם לה' אלוקיכם, לא תתגודדו, it is bad manners to display excessive grief over the loss of a relative as long as a more relevant relative (G’d) is still alive. This is why G’d reminds us in this context that we are His children, i.e. that whatever relative we may mourn we have a father who is alive and well so that we are not really orphaned.

ולא תשימו קרחה בין עיניכם על מת כי עם קדוש אתה, there is no reason to mourn excessively for the person who has passed as he, at least, has not experienced any loss, on the contrary, he has been promoted to the region of eternal life, something of which our sages in Avot 4,6-7 have said that a single hour of the serenity experienced in that life is worth all the combined delights ever experienced during one’s transient life on earth.

בנים אתם לה' אלוקיכם, “You are (as) children for the Lord your G–d.” [This is an introduction to what follows. Ed.] If we are perceived as children of G–d when any Jew dies, the surviving children are not really orphans as G–d never dies, and therefore mourning rites have to take that into consideration. This is why certain mourning rites spelled out in this paragraph as being practiced by the gentiles are not appropriate for Jews. The prophet Jeremiah 2,27, refers to the gentiles as people who because they worship idols made of wood are in fact mourning the loss of their father, and are calling it as such, just as they treat a carved stone idol as if it had given birth to them.