"God told Moses: speak to the priests ('cohanim'), the descendants of Aaron. Let no (priest) defile himself (by contact) with a dead soul among his people." [Leviticus 21:1]
Why are cohanim not allowed to come in contact or be in the same room as a dead body?
Why does the Torah refer to the dead person as a "dead soul"? It is the body that dies, not the soul!
In his book on death and mourning ("Gesher Hachaim"), Rabbi Tokachinsky explained the Jewish view on life after death with the following parable:
Twin brothers, fetuses in their mother's womb, enjoyed a carefree life. Their world was dark and warm and protected. These twins were alike in all aspects but one. One brother was a 'believer': he believed in an afterlife, in a future reality much different than their current, miniature universe.
The second brother, however, was a skeptic. All he knew was the familiar world of the womb. Anything besides what he could feel and sense was only an illusion. The skeptic tried to talk some sense into his brother. He warned him to be realistic, but to no avail. His naive brother insisted on believing in an extraordinary world that exists after life in the womb, a world so immense and fantastic that it transcends their wildest imaginings.
The months passed, and the fatal moment arrived. Labor began. The fetuses became aware of tremendous contractions and shifting in their little world. The freethinker recognized that 'this is it'. His short but pleasant life was about to end. He felt the forces pressuring him to go down, but fought against them. He knew that outside the womb, a cruel death awaited, with no protective sack and no umbilical cord. Suddenly, he realized that his naive brother was giving in to the forces around them. He was sinking lower!
"Don't give up!" he cried, but the innocent one took no heed. "Where are you, my dear brother?" he called out. He shuddered as he heard his brother's screams from outside the womb. His poor brother had met his cruel fate. How naive he had been, with his belief in a bigger, better world!
Then the skeptic felt the uterine muscles pushing him out, against his will, into the abyss. He screamed out ...
"Mazal Tov!" called out the doctor. "Two healthy baby boys!"
Rav Kook wrote,
"Death is a false illusion; its defilement is due to its deceptive nature. What people call 'death' is in fact the intensification of life. Because man wallows in pettiness, he pictures this increase of life in a pained, black fashion, which he calls 'death'."
The cohanim in their holiness are able to rise above this falsehood. Yet, deception rules the world. In order to overcome the illusion of death, the cohanim must limit their exposure to death. They need to protect themselves from those sights that impress the soul with deceiving messages.
The word 'soul' in the verse does not refer to soul of the dead person. It refers to the soul of the cohen. This is how the verse should be understood: "For the sake of the soul, the cohen shall not defile himself among his people." For the sake of the cohen's soul , he must distance and protect himself from death and its illusions.
[Orot HaKodesh 2:380]