The major focus of the second half of Parshas Sh'mini is the identification of the Kosher and non-Kosher species. These laws regulate the consumption of animals, birds and fish as well as insects[1]. Outside of the Land of Israel, with the exception of 'orlah (the fruits produced during the first three years after the tree took root), all fruits of vegetables are Kosher. Even in Eretz Yisrael, where T'rumos and Ma'asros must be separated before fruits and vegetables are consumed, it would appear that the fruits are prohibited (tevel) before this separation in order to assure that the separation is done. Some Rishonim even maintain that the nature of the prohibition of tevel is that t'ruma and ma'aser are mixed in (see Peirush HaRosh to N'darim 12a). As such, the non-Kosher status of tevel fruits is fundamentally different from that of non-Kosher animals. The former is not intrinsic to the fruit; the latter is intrinsic to the animal.
The Kashrus laws are categorized among the chukim, the mitzvos whose reasons were not directly revealed. As the Talmud Yoma 67b states, "'Ani Hashem chakaktiv, ein l'cha r'shus l'harheir bahen" - "I have decreed a decree, you have no permission to doubt it." The literal translation of of "l'harheir bahen" is "to think about them." At first glance, this would imply that attempting to give reasons for the chukim is inappropriate. Nevertheless, we find that the Rishonim did attempt to give reasons for the chukim. Even regarding the paradigmatic chok, that of the Para 'Aduma, the Red Heifer, Rishonim have attempted explanations (see Rashi, Parshat Chukat and Seifer Chinuch). Apparently, they understood "l'harheir bahen" as we have translated it above. One is not permitted to doubt the authenticity of the commandment, but one can and perhaps even should seek out various messages which can be derived from even the chukim (see Rambam end of Hilchos Me'ila and Ramban (Ki Teitzei 22:6)). The context in which this statement appears - that of the Nations of the World or the Satan deriding the Jewish People for keeping seemingly irrational laws - would seem to verify this translation.
Rambam, Ramban and other Rishonim, based on Midrashim write that strict observance of the Kashrus laws allow one to come closer to Hashem, instill religious belief, and lead to success in Torah study. Their violation leads to timtum haleiv, or a spiritual clouding of the mind. (See Kashrus: Much More Than Do's and Don'ts by R. Benjamin Yudin.) Specifically, beasts and birds of prey were prohibited as consumption of their meat would lead to an improper spiritual outlook. Although many Rishonim highlight that they are physically harmful, Rav S. R. Hirsch and others reject this approach and stress the spiritual danger caused by their consumption. Rav Hirsch (in his Horeb) adds that even animals like the hyrax and the mouse who are herbivores and not carnivores were prohibited since they are extremely active creatures symbolizing a general excessive pre-occupation with the purely physical side of the world. Contrasted with them are the Kosher cow and sheep, docile farm animals who inform us, that although we must engage the world, we should not be totally pre-occupied with its physical side.
Perhaps we can add the following insight into the Kashrus laws. The original diet of mankind and of animals as well was only from the vegetable kingdom (see B'raishis 1:29-30). Only from the post-Flood era and onward was the consumption of animal meat permitted by G-d (see No'ach 9:3). Some suggest that this indicated a lowering of the level of mankind in the post-deluvian era which manifested itself in a physical manner as well. Man's and animal's nutritional requirements would generally now have to be filled from animals and not exclusively from plants. Whereas before the flood, killing even beasts of the field for food was prohibited, now it was permitted. The Torah therefore places many restrictions on this additional mastery over the animal inhabitants of the world. For the general populace, only eiver min hachai (meat from a live animal) was prohibited. For the Jewish Nation, additional restrictions, both in terms of the species consumed and the manner in which the animal is slaughtered, were given. These restrictions therefore would serve as a safeguard that Man not become conceited and view himself and the true master of the world. It is not surprising that extremely haughty figures in history such as Nebuchadnezzar violated even the laws of eiver min hachai.
As mentioned above though, although approaches have been given to explain the bulk of the Kashrus laws, at their core they remain chukim. Indeed, as Rav Yosef Dov HaLeivi Soloveitchik zt"l - the Beis HaLeivi - explains, even those mitzvot whose reasons are explicitly stated by the Torah such as that of Korban Pesach, are, at their core, chukim whose fundamental reasons defy human understanding. Hence, the Torah refers to "Chukat HaPesach". Just as G-d himself is ultimately hidden from Man's mind although glimpses of His Presence and interaction in the world can be studied (see Ki Tissa 33:12-23 Malbim on Tehillim 145:1), so too the ultimate reasons for His mitzvot remain a mystery although we can learn a plethora of practical spiritual lessons from them. Perhaps the ultimate spiritual benefit of greater understanding of G-d's Wisdom that accrues from adherence to the Kashrus laws reflects the fact that those who keep them demonstrate their subservience to the Higher Power who commanded them notwithstanding our lack of ability to explain them fully.