Kedoshim yihyu lai’elokaihem ... and they shall be consecrated to their G-d (Vayikra 21:6)
Ramban notes that the obligation of the kohanim referred to here parallels the requirement that was delineated in the first pesukim of parashas Kedoshim which was incumbent on the entire nation. Just as there the admonition was to separate from that which would otherwise be permitted, so too here the kohen is required to accept another level of separations; e.g., he is required to refrain from coming into contact with the dead and to accept another layer of forbidden marital relationships above that required of all other Jews.
Ramban’s comments are somewhat difficult to understand, for in the case of the kohanim, the steps or actions necessary to reach kedushah are mandatory, whereas in his opinion (see parashas Kedoshim), the parallel level of kedushah that the Torah suggests that every Jew aspire to are made up of voluntary actions; i.e., perishus - separating oneself by choice from that which would otherwise be permitted.
Moreover, the Torah clearly specifies - as Ramban notes - the steps necessary for a kohen to achieve kedushah. He must not defile himself by coming into contact with a corpse other than specified close relatives (21:1), he may not shave his entire head or cut himself as a sign of mourning (:5), and he may not marry a zonah, chalalah or a divorced women (:7). The kohen gadol, by virtue of his elevated status, is also forbidden to allow his hair to grow or to rent his clothing as a sign of mourning (:10), come into contact with any corpse (:11), or marry a widow (:14). If we take the kedushah of am yisrael to be parallel to that of the kohanim, albeit on a lower level, should one not expect that the laws delineated after the Torah’s mention of kedushah of am yisrael would call for a level of separation. For example, would it not be logical to place the laws of kashrus, or the laws of not following in the practices of the nations (18:3) in juxtaposition to the requirement to be kadosh, since these are the type of separations that might parallel the separations of the kohen and make us distinct?
Furthermore, as we have already noted in our comments to parashas Kedoshim, many of the mitzvos delineated there are unrelated to rituals. The portion immediately adjacent to the admonition of being kadosh teaches the laws of tzedakah (19:9-10), thievery, denial and lying (:11), swearing falsely (:12), not paying an employee on time (:13), misleading people (:14) as well as many other interpersonal mitzvos. In what manner are these mitzvos connected to achieving a level of kedushah?
I would like to suggest that there are two levels of kedushah referred to in these parshiyos - one individual and one communal. Parashas Kedoshim deals with the latter, and as such the mitzvos taught there reflect a standard wherein all of the community accepts a level of behavior that reflects a commitment to the public welfare at the expense of furthering individual agendas. For example, the laws of leaving the portions for the poor, teach man that tzedakah is not an act of voluntary sympathy, but rather an obligation. I must pay my employee on time because I have to understand that when a person is dependent upon me, I have obligations to them. The mitzvos in Kedoshim are chosen to create an attitude where the individual understands his obligation to the klal and makes his choices in life accordingly.
It is only when that mind frame is created that one can go on to the next level of kedushah, creating special laws for individuals. The extra level of sanctity demanded of the kohen is only possible once the klal as a whole has inculcated the concepts of community responsibility that the Torah teaches in Kedoshim. Man cannot hope to achieve the level of sanctity demanded of a kohen if he is deficient in his relationships with other men. Thus, man can only be sanctified by the avoidance of tumah, if he has first accepted the avoidance of lashon hara (:16) and of standing by watching his neighbor suffer (ibid.). The kohen who has not yet accepted the responsibility of admonishing his fellow Jew so as to help him avoid further sin (:17) can not successfully offer karbanos as a means of bringing that man atonement.
A note to us as educators: then Torah first speaks of the kedushah of the community and only then goes on to the kedushah of the individual. Perhaps we would do well in emulating this order when setting priorities in our schools and invest a little bit more in teaching those mitzvos which may be bein adam lechaveiro but bring us to kedushas haklal.