Humans are the only species to continue to drink milk into adulthood, obviously because milk's sole natural purpose is for the nursing of infants. If you don't continue to drink it with much regularity, your body stops producing the enzymes needed to digest it properly (one of the reasons for lactose intolerance) since there's no real nutritional reason for you to keep drinking it (some debate the merits of using it to rehydrate after a workout, and cite the long chains of amino acids as assisting in building muscle, but there's really nothing you get from milk that you can't get elsewhere, in certain cases in much more bio-available forms, i.e. more readily usable by the body). But milk is not a universal substance, and it differs compositionally between animals. Milk from a human female is actually higher in lactose than cow's milk, and higher in fat. Cow's milk has minerals human milk doesn't, and is higher in protein. Different breeds of cow also produce milk that differs from other breeds. And, of course, unless you're getting it from a consistent local source or are paying for better quality organic stuff (and even then you're really not guaranteed anything because many agricultural companies lie pretty much constantly about basically everything), it's going to have residual hormones and antibiotics in it. And no matter who you get it from, it's a biologically produced substance from mistreated animals forced to eat feed made from all kinds of bad shit, most especially renderings of other ground up cows (legislation hasn't stopped this practice entirely), euthanized shelter animals, roadkill, dead horses, pigs, chickens, and turkeys, fecal matter from chickens, and bovine blood and fecal matter. This all varies from place to place, but this is what rendering plants produce, and even if you're not drinking milk that comes from cows fed all that, they're usually still fed other bad stuff like cheap, shitty, genetically-modified soy, corn, and other stuff that isn't very good for them (especially given that both of these are the crops most overloaded with pesticides; along with cotton and wheat, they get about 80% of the pesticides used in the US). Hell, they can even get stuff like DDT remnants that are still in the ground after all these years. And as we all know from nursing mothers of our own species, milk is highly susceptible to contamination, which is why mothers are so careful about avoiding certain things that could cause harm to their babies.
