Dogs roll in grass and other smells to mask their scent before/after hunting so prey doesn't detect them easily.
Scent Masking
Grass feels nice and cool against a dog's skin. Rolling spreads that coolness over more surface area helping lower body temperature.
Cooling Down
Rubbing and rolling presses hard-to-reach itchy spots against grass and dirt, letting dogs scratch themselves. It can feel great!
Scratching
Many dogs simply enjoy and feel great rolling in grass, dirt, mulch as they play, explore, have fun in yards and on walks.
Joy and Play
Smelling new/intriguing smells by rolling arouses dogs' curiosity to further investigate and learn about their environment.
Investigation
Some breeds like Labradors are more prone to rolling and rubbing due to higher energy, more curiosity, or special coat textures.
Breed Factors
Coarse-bladed, tall grasses seem most tempting for allowing deepest rolling and best scratching sensation.
Grass Types