Why is this verse repeated three times (Exodus 23:19, Exodus 34:26, and Deuteronomy 14:21) in the Torah? The Sages of the Talmud say:
Once to teach us that we should not cook meat and milk together. A second time to teach us that we should not eat meat and milk products together. A third time to teach us we should not derive any benefit from this mixture even if it was done inadvertently.
We mustn’t cook a kid in its mother’s milk because that is cruel and insensitive. We could universalize this sensitivity by cooking all mammals, which must be killed, separately from all milk products, which give life to their young. We could go even further and not eat the products of slaughter and the products of nurture at the same meal. We might even go further yet and use different plates, and eating utensils.
The prohibition against mixing meat and milk products together has expanded further and is applied in a stricter way than any other dietary law in the Torah. There are some individuals who even abstain from food that the Torah permits. Rashi says that when we are with these people we should follow their practice, for by abstaining from that which is actually permitted we can attain a higher level of holiness.
But doesn’t this line of thinking violate the Mitsvah (repeated twice in Deuteronomy 4:2 and 13:1) that you should not add to the Mitsvot? Yes it does!
By continually expanding the strictness of mixing meat and milk products you end up with separate dishwashers for meat and dairy dishes. The Orthodox say the prohibition against adding to the Mitsvot applies to literal adding (2 days of Yom Kippur or 9 days of Hanukkah) and not to extending the application.
Yet they add another hour to Shabbat making it last 25 hours, and celebrate Rosh HaShanah for 2 days even in the Land of Israel. The orthodox also added poultry to the prohibition of mixing meat and dairy although poultry do not produce milk.
Fortunately the Orthodox did not add fish to the forbidden meat/dairy mixture because of an idea in the Talmud that eating fish and meat together is unhealthy.
But if you can’t add to the Mitsvot how can you adapt the rules to new conditions and situations? By following the whole Mitsvah, which is not to add or subtract i.e. do not add unless you subtract elsewhere else. Do not subtract unless you add elsewhere else.
Thus the Torah will not become ridged but will always be balanced and flexible. We should not teach future generations, or judge our own generation, either to the far right (constantly strict) or to the far left (constantly permissive).
The Mitsvah to avoid the far right and the far left is repeated four times in the book of Deuteronomy. Once to teach us not to direct others towards undue strictness or leniency (5:32). A second and third time to specifically teach judges (17:11) and rulers (17:20) the same lesson; and a fourth time to teach all of us to be very careful because both blessings and curses can result from a consistently one sided emphasis for Mitsvot (28:14).