Milk is compared to nectar, which one can drink to become immortal. Of course, simply drinking milk will not make one immortal, but it can increase the duration of one's life. In modern civilization, men do not think milk to be important, and therefore they do not live very long. Although in this age men can live up to one hundred years, their duration of life is reduced because they do not drink large quantities of milk. This is a sign of Kali-yuga. In Kali-yuga, instead of drinking milk, people prefer to slaughter an animal and eat its flesh. The Supreme Personality of Godhead, in His instructions of Bhagavad-gita, advises go-raksya, which means cow protection. The cow should be protected, milk should be drawn from the cows, and this milk should be prepared in various ways. One should take ample milk, and thus one can prolong one's life, develop his brain, execute devotional service, and ultimately attain the favor of the Supreme Personality of Godhead. As it is essential to get food grains and water by digging the earth, it is also essential to give protection to the cows and take nectarean milk from their milk bags.

From Srila Prabhupada's purport to SB 8.6.12