Or HaChaim on Deuteronomy 14:1/אור החיים על דברים י״ד:א׳

1 בנים אתם לה׳ אלוקיכם, "You are children to the Lord your G'd, etc." Why did the Torah write this statement next to the prohibition to make incisions on one's body? It appears that the reason is to teach us that if one loses a family member to death one should not look upon the deceased as having become lost. The true state of affairs can be understood by a parable. A father sent a son to a distant city to buy some merchandise. Seeing the son did not return, the father sends out someone after a while to try and locate him. The son's absence was felt only in his home town; this does not mean that he no longer exists. Moreover, the chances are that the reason he did not return is that he is better off in his new home as he is closer to the original father, i.e. G'd, who was the primary source of his having lived on earth in the first place. This is the reason we must not make incisions on our bodies, or to make bald spots on top of our heads in mourning for the deceased. This law would not apply to the Gentile nations as they are not in the category of being "children of G'd." The reason the Torah stresses the word אתם, "you," is to explain the difference between the death of a Gentile who does indeed cease to live in any world upon his physical death and the Jew who moves on to a higher world.