Another approach to the words אמור אל הכהנים, “speak to the priests.” Seeing that the subject of Moses’ address to the priests concerns the priests’ sanctifying themselves by abstaining from incurring ritual defilement, all the special laws addressed to them may make them feel superior, or even haughty. Moses is to warn them that the fact that a part of the Torah is exclusively addressed to them must not make them feel that they are better or holier than the remainder of their peers. Moses is to first acquaint them with the law that the priests must not defile themselves ritually even when not likely to be called upon to perform their duties in the Temple as part of the weekly roster. Such defilement would gravely damage their נפש, “their essence” as servants of the Lord.
G’d is known to hate arrogance, haughtiness, etc.; in the case of the priests’ being haughty, an additional consideration is that haughtiness when it is the result of someone having accomplished a difficult task, something that he had invested much effort and time in, is still unacceptable as we know from Jeremiah 9,22. If pride or haughtiness is looked upon as a negative attribute in such cases, how much less is it acceptable in people who have been promoted to an elevated status merely by having been born to a father who is a priest, without being able to claim superiority by reason of their personal excellence?
This is implied in the line: אמור אל הכהנים בני אהרן, “say to the priests, the sons of Aaron, etc.” There was no need to add the words: בני אהרן, “sons of Aaron;” we all know that the priests were descended from Aaron. The Torah added these words as a reminder that the priest’s status was not earned, but is hereditary. If Aaron had not been holy, none of his descendants could have claimed this distinction without earning it. They therefore do not have any reason to boast about their superior status.