What i found from the net,

"It is important to wash your pet with shampoo that was specifically designed for your pet. The use of human shampoo on your pet will hurt your pet’s skin, not help it. The main difference between pet and people shampoo is the pH balance, or measure of how acidic a substance is.

Shampoo designed specifically for humans has a pH balance of 5.5. Pet shampoo has a pH of 7.5. On the pH scale, seven is neutral, while anything below that is more acidic. The more acidic shampoo is both good for humans, and helps to keep our hair healthy. However, this level of acidity is harmful to pets. This can be compared to babies. Parents use shampoo specifically designed for babies because adult shampoos are too harsh for a baby’s soft, delicate head. Likewise, adult human shampoo is too harsh for your pet’s gentle skin and fur.

Pet shampoos are specifically formulated to be gentle on your pet’s skin. These products also help to nourish your pet’s fur to give it a healthy, shiny appearance. Another important reason to use pet specific shampoo is that it moisturizes the skin. Healthy skin will both help prevent your pet from getting skin problems and improve your pet’s overall health...."

It does make sense though