Parsahat Shemini introduces the details of kashrut. When broad principles of ethical and decent living are given, it is normally clear they have a wide ambit.  At the end of the listing of these laws, we are told that they come from Hashem, who elevated us from Egypt. 

Implicit in this is that they are part of our national covenant.  Perforce, the laws of kashrut are to be understood on a spiritual, and not a medical, basis.

This proposition is reinforced by a statement by the great commentator Rashi, who observes that when we pass a non-kosher butcher shop, we ought not shudder in disgust at the treif meats on display within; rather, we should acknowledge that others are permitted to eat these foods, but we are required to distance ourselves from them.

Keeping kosher, alone, does not make a person good. The rabbis admonish that eating strictly kosher but in a gluttonous manner is despicable and unholy.
 
In calling on us to make distinctions, the Torah hints at one of the underlying purposes of kashrut.  The Torah is demanding of us awareness. We are supposed to consider the origin of our food; not just grab or wolf down whatever lies before us. Indeed, that consideration should extend to the ethical sourcing of our foods, the treatment of livestock and the people involved in production.

Moreover, in our diet as with much in our life, we are supposed to exercise self-censorship despite our tastes.  

We are reasonably adept as masters of our lives and live in a society where we like to be able to define what we want and to get it… and to get it now. The laws of kashrut remind us that we do not have unfettered freedom: “I am Hashem who elevates you from the land of Egypt to be your Hashem. You shall be holy because I am holy.”  

Even on the most mundane level of what we eat, Hashem calls on us to acknowledge that we are a part of a people, answerable to our redeemer and charged with a noble purpose.