Teeth get discolored from the food that you eat and the things that you drink. Almost everything you eat or drink is a color and over time that can stain your teeth, this is why brushing is so important.
I believe that Humans drink the most milk in their lifetimes. Far as I know, we are the only species that continues to drink milk after infancy. Cats and dogs will drink milk after infancy. It is not necessarily good for their digestion or teeth, but I have seen them drink it first hand.