Yes milk is the most important healthy diet I guess. As milk is probably the first diet consumed by all the human beings as well as animals. So mother milk is the first diet taken by infant via breast feeding.

Also during young age - thats why every now and then mom always insist on their child to have milk.  (Thats basically cows or buffaloes milk) Humans generally feed mothers, buffalo, cow, goat, sheep and camels milk. Also people have various milk products such as ghee, milk, cheese, paneer, different sweets, chocolates etc.

So whats the reason - why milk is used universally.

* Generally its believed from generations to generations that a child who regularly has milk (since childhood) is healthy and strong - and his/her immune system is also stronger as compared to other children.
*  its prevents hypertension, osteoporosis and colon cancer - since your immune system is strong
*  Reduce tooth decay