I think, (not sure) it's because they are itching their backs
Dogs roll in certain parts of grass to mask their scent with the smell of the grass or to mark their territory with their own scent.
Dogs roll in the same spot of grass repeatedly to mark their territory with their scent and to mask their own smell to help them blend in with their environment while hunting.
Dogs roll in grass to mask their scent and communicate with other animals. This behavior is instinctual and helps them blend in with their surroundings while also leaving their own scent behind as a form of marking territory.
no dogs tongues do not roll
They smell because they are looking for a good spot to relax and take a break from all the trouble they've done in a day.
dogs mostly roll when wanted to be petted or wants to play.
Because they're dogs...
In the grass
Dogs eat grass to help digest their food.
A ball will typically roll faster on dirt compared to grass, as dirt provides less resistance and friction than grass. Grass can create more friction which can slow down the ball's movement.
A ball would likely roll farther on grass than on carpet, as grass provides a smoother surface with less friction compared to carpet. The longer blades of grass can also help propel the ball further by reducing resistance.
Dogs perceive grass as shades of yellow and blue due to their limited color vision.