Any man who is a patrilineal descendant of Aaron, the brother of Moses, is deemed to be a kohain, a member of the priestly family of Israel. Aaron and his sons were called upon by God to take charge of the service in the Tabernacle, and later the Holy Temple in Jerusalem. In exchange for this great honor, however, the priests were also given more restrictions in their day-to-day lives than the other Children of Israel.
Because the kohanim (priests) worked in the Temple, it was necessary that they avoid carrying any spiritual uncleanliness into the Temple boundaries. While it is virtually impossible to explain the concept of spiritual cleanliness or uncleanliness within the limits of a Jewish Treat, it can be stated that association with death and dead bodies is one way to acquire spiritual impurity (tumah). A kohain is therefore forbidden from touching a corpse or being in an enclosed area where a dead body is present.
At first glance, this seems to be a rather simple rule. After all, not many people are excited about dead bodies. But what about funerals? A kohain may not attend any funerals, with the exception of: his mother, father, son, daughter, brother; unmarried sister and wife. And, in the times of the Holy Temple, the Kohain Gadol (High Priest) was not permitted to attend any funerals at all.
While we no longer have the Temple service, the prohibition still remains in effect, as does the elevated status of the kohain (who is often given special honors during rituals and services).
For possible reasons for these priestly restrictions, read Rabbi Buchwald’s parasha analysis in the following articles:
”Death, and the Kohanim--the Children of Aaron"
"Priests and Death: An Unusual Relationship"