Just like the type of shampoo that people use can differ from person-to-person, the same applies to dogs. Why? Well, it is because dogs have different skin types just like human skin types differ from each other. Using the wrong shampoo can cause skin irritation and this can cause quite a bit of confusion when trying to find the right shampoo.
And do not think that using a human shampoo on a dog is safe because human shampoo does not have the same acidic pH that a dog shampoo does. The dog can experience skin irritation because the human shampoo has a pH that is lower than what the dog's skin needs.