In the beginning of parshas Tzav, we learn about the mitzvos of terumas hadeshen, clearing the altar of the ashes, and hotza'as hadeshen, removal of the ashes from the Mishkan. The kohein is told to raise the ashes which the fire consumed and place them next to the altar. Then he is told to change his clothing and move the ashes to a pure place beyond the camp. Rashi writes that the changing of the kohein’s clothes is actually not an obligation but rather a matter of derech eretz [proper conduct] in order that the kohein avoid soiling the clothing he was wearing in the Mishkan. As an analogy, Rashi says that when a servant pours a cup for his master, he should not wear the same garments that he wore when cooking a pot. Why, though, does Rashi consider this merely derech eretz and not obligatory?
Actually, there is an earlier source in the Torah for changing one's clothes between different activities done for a respected person. In parshas Toldos, when Rivkah prepares Yaakov to impersonate Eisav, she tells him to wear Eisav's clean garments (Bereishis 27:15). The midrash (Bereishis Rabbah, 69) explains that Eisav would always change his clothing before serving his father. Rabbi Shimon ben Gamliel commented that although he himself served his father all his life, he did not serve him to even one hundredth of the degree that Eisav served Yitzchok. Whereas Rabbi Shimon ben Gamliel would serve his father in his soiled clothing and then change before going outside, Eisav, would change his clothing before serving his father out of respect.
Rabbi Yeruchom Levovitz, the mashgiach ruchani (spiritual mentor) in the yeshiva of Mir in Europe, raises a question on this midrash. Since R. Shimon appreciated the magnitude of Eisav's respect for his father, why didn't he also change his own clothing before serving his father? Rav Yeruchom answered that although R. Shimon could easily have changed his clothing as Eisav did, he did not feel that he was on the level to exhibit that kind of honor. Eisav's changing to special clothing to serve his father was a reflection of his general treatment of his father and his relationship with him. Rabbi Shimon ben Gamliel felt that his level of honor for his father fell short of Eisav's. Had he changed his clothing to serve his father in the way that Eisav did, it would have been an artificial, meaningless act, a mere aping of Eisav's comportment but not a true reflection of his own relationship.
Based on Rav Yerucham's analysis of the midrash, we can understand why Rashi says that it was not obligatory for the kohein to change his clothing. Rav Yerucham writes that Eisav’s actions toward his father reflected his internal level of respect. Similarly, the kohein's actions, and the clothes he wears, must represent his internal state. Thus, if he changes his clothing between the terumas hadeshen and the hotza'as hadeshen, he must do so because he actually senses that it is not proper for him to wear the same set of clothing for both acts of divine service. Therefore, the Torah could not make it obligatory. The kohein's actions had to reflect his inner connection to God, just as R. Shimon ben Gamliel felt that his outer comportment toward his father had to reflect his inner connection. By extension, there is a message here for all of us, in our own divine service. When considering the adoption of practices that go beyond what is strictly required, we must be sure that these practices reflect our true relationship with God, and are not merely an artificial imitation of someone else's actions. Otherwise, we may actually be serving ourselves, rather than serving God.