“You are the children of HaShem your G^d; don’t cut yourselves, nor make baldness between your eyes for the dead” (Devarim 14:1). The second half of the verse forbids a pagan mourning custom of tearing out the hair; it is clear that “between your eyes” actually refers to the hair above the forehead (SA YD 180:9-12).
Several Rishonim explain that this mitzva is the basis for the directive of our Sages forbidding excessive mourning (Ramban and Chinukh, as ruled in SA YD 394). The usual understanding is that excessive mourning shows a lack of faith in HaShem’s providence and in the World to Come.
Rav Natan of Breslav in Likutei Halakhot on this mitzva adds an additional consideration: excessive mourning involves a preoccupation with death. “Thus we are commanded not to grieve too much over the dead, so as not to invite the aspect of death, G^d forbid; rather, we need to put it out of our mind and forget it.”
Rav Natan explains that a defining characteristic of pagan worship is that it reconciles itself to the reality and importance of death. This contrasts with the Torah, which is a Torah of life, affirming the reality and ultimate sanctity of life in this world. (This is explained in depth in Rav Kook’s Orot HaKodesh vol.II p. 488.) As a result, idolaters are constantly concerned to remind themselves of death, and when they mourn they strive to make a permanent reminder. “Idol worshipers, who are far from sanctity… are from the Other Side, the side of death, therefore they want to augment, G^d forbid, the Other Side, and thus they make baldness or cuts on their dead in order to remember them.”
(This approach is strengthened by the fact that this law primarily prohibits a permanent uprooting of hair, just as cutting the flesh creates a permanent scar and the related prohibition on tattoos forbids a permanent mark in the flesh.)
We explained last week that the expression “between your eyes” regarding the head tefillin also refers to the scalp above the hairline, and that this is learned from the prohibition on tearing the hair in mourning. Rav Natan explains that these commandments are intimately related: we are constantly concerned to remind our-selves of holiness, thus we bind tefillin specifically to the top of our heads, the place of our highest faculties; we may point out that part of the mitzvah is not to distract ourselves from their presence (SA OC 37:2). Idol worshipers choose this exact place to make a remembrance of death.
Let us sharpen this contrast a bit. The Torah also commands the mourner to make a permanent reminder of his or her loss, by tearing the garment. The tear for a parent may never be permanently mended (YD 340:15). Yet the true symbolism of this mitzva is exactly the opposite of preoccupation with death; the deeper message is one of renewal. The ultimate result is that the old, torn garment is put aside and a new garment is acquired. Thus, the prohibition on mending the original torn garment is an impetus to make an even more thorough renewal than would otherwise take place.