a) The mercantile class is required to give protection to the cows in order to get sufficient milk and milk products, which alone can give the proper health and intelligence to maintain a civilization perfectly meant for knowledge of the ultimate truth. (Swami Srila Prabhupada SB 2.5.37)

b) The body can be maintained by any foodstuff, but cow’s milk is most essential for the development of the finer tissues of the human brain. Which is essential for understanding transcendental knowledge. (Srimad-Bhagavatam)

Basically without drinking cow’s milk, the humans will eventually become zombies. With no ability to understand and know God as a real Person.

c) It is the Vedic teaching that the cow is called a mother, because her milk is drunk.

d) Everything is related to cows.

Weather

Crops

Rain

Human health

Oxygen

Wars

Natural disasters (floods, droughts, earth quakes, hurricanes etc.)

Human mentality

In the body of every cow is the potency of the 33 categories of demigods, which number in the millions. The demigods maintain everything in the universe.

Cow Killing = Insult to demigods = maintenance is disrupted = everything falls apart

e) Cows milk is called the most complete and perfect food, because it contains all kinds of nutrients that are needed for good health.