Milk often contains unwanted ingredients.
Under current industrial methods, cow's milk is often a toxic bovine brew of man-made ingredients like bio-engineered hormones, antibiotics (55% of U.S. antibiotics are fed to livestock), and pesticides—all of which are bad for us and the environment. For example, unintentional pesticide poisonings kill an estimated 355,000 people globally each year. In addition the drugs pumped into livestock often re-visit us in our water supply.
Which brings us to... health reasons to avoid milk.