Rubbing their body against a bed of soft grass makes your dog happy. It is a kind of relaxing therapy for your dog to channel its energy in a cheerful way.
Your dog is frequently rolling in grass, carpet, or ground because its back is itchy or it has skin infection.
Your furry pal sometimes rub its back against the grass to shrug off dirt and from inaccessible parts of their body like ears.
The shampoo smell that you like is actually an odour for your dog and so it is rubbing its body to get rid of it.