What Is Kosher?

Great question. The truth is there's plenty of kabbalah on this topic... but after all said and done, although there may be reasons for it, it's not why we'll do this mitzvah, but rather because its a commandment from an infinite G-d, connecting us to that infinity.
The infinite bond with g-d is of most importance here (same for all mitzvot, furthermore by a Mitzvah that g-d didn't give a reason, how much more powerful is a connection not based beyond reason). Reply

When the milk is in the cow, the cow is still alive. If the cow is milked, and then killed or its calf is killed, it isn't kind (or kosher) but cruel to cook the cow (or it's calf - since that is why it is giving milk) in it's own milk. These days it is unlikely to be the same cow, calf and milk because of the large herds, but in small farms it could easily have been the same. In addition, it evidently was a special culinary practice of the heathen neighbours to prepare such dishes. Judaism teaches kindness to animals and eating milk and meat together engenders callousness. Reply

The Torah tells us that meat cannot be cooked with milk. This is forbidden by the Torah. Not mixing milk & meat (eating them together etc.) Is a rabbinic law that is to prevent us from coming too close to the forbidden action of cooking them together. It is a guard to keep us safe. Reply

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