No one really knows why dogs eat grass. The majority of experts agree that it is not a cause for concern.
Many dog parents will notice that their dogs occasionally graze on grass while out on a walk or in the backyard. Most of the time, there is very little cause for concern for this behavior. Wild canids are frequently seen eating grass in their natural habitat.
Some people believe that their dogs eat grass due to a nutrient deficiency in their diet. If your dog is on a well-balanced diet, appropriate for their life-stage and formulated according to AAFCO guidelines, there is little concern that your dog has a nutrient deficiency.
Others believe that dogs only eat grass when they have an upset stomach. Occasionally, dogs will eat grass and then vomit afterward (whether the dogs do this on purpose or not is up for debate). Dogs definitely do not only eat grass if they are feeling sick, but if your dog consistently eats grass and vomits afterward, then they should be evaluated by a veterinarian.
In summary, if your dog sometimes grabs a few blades of grass out on a walk, there is little cause for concern. If it becomes an obsessive behavior or there is vomiting associated, then he or she should be evaluated by a veterinarian.
Now, if your dog eats grass, marijuana intoxication is a whole different matter.