"....
According to the cosmetic chemists, detergent is actually better for your hair than soap. Soap solutions are alkaline, which makes the cuticle cells swell up and get rougher. That leaves h
air dull looking. In addition, soap may leave behind deposits of calcium and magnesium (known as soap scum), which will also make the hair dull. Old-fashioned beauty books suggested completing your shampoo with a lemon juice or vinegar rinse. Acid helps dissolve soap scum and makes the cuticle scales lie flat, protecting the cortex and making your hair shiny and smooth. Most shampoos and conditioners are slightly acidic, for the same reason..."
source:
http://www.exploratorium.edu/exploring/hair/hair_2.html
I think this article can help you understand better, check all the ingredients on the shampoo bottle. Some "Natural shampoos" are soap based which is pretty bad for your hair.