Humans are 'designed' (if we were actually designed I want to kick the designer in the nuts
) to drink milk. Breast milk that is. Milk is one of the moust nourishing drinks there is and has lots of vitamins and stuff in it that a growing baby needs.
A long time ago it was discovered that other animals milk can also be consumed by people and can give a lot of nutrients to us as well, provided that we are able to digest it. We have an enzyme in our digestive system called Lactase. We lose this enzyme naturally as we grow up, and we lose it by not drinking milk. If you keep drinking milk as you grow up you retain lactase, if you stop drinking milk then you lose lactase, and if you have no lactase then you are 'lactose intolerant' (what happens is the lactose in the milk gets deeper into your digestive system without being broken up by the enzyme and the bacteria etc that live lower down eat the lactose and multiply in places where they shouldn't)