Emor 5780

The parsha starts off with the laws of a Kohen becoming impure for certain deceased relatives. The Kohanim were always in a state of purity as they carried out the services in the Temple. However, the exception to the rule  The Gemara in Moed Katan (20b) explains that for whomever they become impure, they also must mourn for them. The list given is discussed in the Gemara in Yevamos and applies to all Jews and not just the Kohanim.
The Rambam lists this as a Mitzvah in Sefer Hamitzvos (Aseh #37) that the Kohanim are obligated to become impure for their family members. He explains that because they were already prohibited from becoming impure via deceased individuals, they would understand the Torah here as giving them the option to do so for family. Therefore, the Torah *obligates* them to do so, even if by force. Additionally, the Kohanim are singled out to further strengthen the idea of mourning. By singling out the Kohen, who is cautious of impurity, to become impure and mourn, the Torah sends the message to the rest of the nation to not be weary with the laws of mourning.