First Reason: Dogs have a natural instinct to eat a variety of foods and this also include eating grass.
Second Reason: Eating grass can be a way for dogs to do vomit and this make their stomach clean.
Third Reason: Eating grass could be an reason if your dog lacking proteins and want to get them from grass.
Fourth Reason: eating grass might suggest you that your dog is having boredom or anxiety.
Fifth Reason: Some dogs may find the taste or texture of grass appealing and they eat grass.