Milk is indeed a source of protein and calcium that some people do well on. But many adults have problems with one or more of its components. As they grow out of childhood, they stop making the enzyme that breaks down lactose during digestion. When lactose-intolerant people drink milk, they experience digestive upsets such as gas, cramps, or diarrhea. Butterfat, the fat in milk, is the most saturated fat in the American diet.
Milk fat is a principal contributor to high cholesterol and artery-clogging atherosclerosis.
The protein in milk, called casein , irritates many people's immune systems. This is the component of milk that stimulates mucus production. Casein is often associated with allergic conditions such as recurrent ear infections in early life, eczema, chronic bronchitis, asthma, and sinus conditions. Most commercial milk also contains residues from drugs, hormones, and chemicals used to keep modern dairy cows producing abundantly. I think most people should limit their intake of whole milk and the products made from it.