The Midrash Rabbah makes an intriguing statement: Any nation or people which “elevates” itself over Hashem or His nation will be judged with fire. Chazal are teaching us the terrible punishment in store for the person, or people who arrogate personal eminence – who views himself or themselves as greater than Hashem. While this is addressed primarily to the gentile oppressors who feel that they can take on Hashem and His people, it also serves as a blanket statement decrying the ill effects of arrogance. Let us face it: Whoever acts haughtily is, in effect, against G-d, in Whose eyes we are all the same. One question remains to be addressed: Why of all places in which the Torah could have renounced the baseness of arrogance, does it choose to do so while speaking to the Kohanim? What do Kohanim have to do with gaavah, haughtiness? They are the symbol of the ovdei Hashem, those who serve Hashem, with extreme devotion. They should be the last ones to whom the effects of this disgusting character trait should be addressed.
The Chasam Sofer, zl, explains that it is specifically concerning Kohanim that this character defect is so reprehensible. The Kohen has nothing about which to glorify himself. He has no property, no major possessions. He is supported by the nation, while he devotes his life to serving Hashem. It is not unusual for a person of means to allow his material abundance to go to his head. The Kohen, however, has no material abundance. He serves the people. Why is his head up in the clouds?
Thus, a Kohen who is arrogant is especially detestable. In the Talmud Pesachim 103, Chazal say that the mind cannot tolerate a pretentious, poor man. His pomposity is especially abominable because he has nothing. He is no different than anyone else. Let him act that way.