Now I'll tell you why.
Many people like to keep weaned dogs and then feed them with milk. As we all know, cows are herbivores, while dogs are omnivores. Milk is very different from dog milk. Milk is low in protein, low in fat, high in lactose and high in phosphorus and potassium, while dog milk is just the opposite, high in protein, high in fat, low in lactose and low in phosphorus and potassium. Dogs lack lactase that can digest high lactose, and frequent feeding can easily lead to diarrhea in dogs. At the same time, the protein content of milk is only 1/3 of that of dog milk, so puppies can't get enough nutrition. Dogs that are still breastfeeding have low immunity, and long-term feeding with milk can easily lead to premature death of dogs, but most people still don't know why.
If the adopted dog has passed the lactation period, there should be no problem in this regard. But it should also be noted that it is best to feed it occasionally, even if our dog is interested in the milk given.
Puppies are advised to drink their mother's breast milk. If not, they should buy special dog milk powder, which is sold in large pet stores and fed according to the dosage on the milk powder package.