Why do we keep Kosher?

Keeping kosher is a mitzvah, a divine "commandment" and "connection." We eat kosher because G-d commanded us to, and by fulfilling the divine we connect to G-d.
Our sages also point out the various advantages of the kosher laws: the health benefits, the humane treatment of animals, their unifying effect on a dispersed people, and their role as shield against assimilation. Nachmanides, the great 12th century sage and kabbalist, points out that "the birds and many of the mammals forbidden by the Torah are predators, while the permitted animals are not; we are instructed not to eat those animals, so that we should not absorb these qualities into ourselves." Kashrut can thus be seen as "spiritual nutrition": just as there are foods that are good for the body and foods that are harmful, there are foods that nourish the Jewish soul and foods that adversely affect it.
None of the above, however, are "reasons" we keep kosher. Rather, the reverse is true: because it was commanded by the Creator of our bodies and our souls, the kosher way of life will obviously be beneficial to both.