As we saw above, the Torah prohibits extreme forms of mourning the death of loved ones. Since the laws of nature dictate that every living thing will eventually die, why is human nature to mourn the loss of a loved one, sad as it may be, with such intensity when we mentally recognize that death is inevitable?
Citing the Ramban’s work Toras HaAdam on the laws of mourning, Rav Yitzchok Hellman offers a fascinating explanation for this phenomenon. When Hashem created the first man, Adam, He intended him to be immortal and created him with a nature reflecting this reality. When Adam sinned by eating the forbidden fruit, he brought death to mankind and the entire world.
Nevertheless, although this new development completely changed the nature of our life on earth, it had no effect on man’s internal makeup, which was designed to reflect the reality that man was intended to live forever. Although our minds recognize that people ultimately must die and we hear about death constantly, our internal makeup remains as it was originally designed. We expect our loved ones to live forever as they were originally intended to, and we are therefore plunged into intense mourning when confronted with the reality that this is no longer the case. (R’ Ozer Alport)