Dogs commonly eat grass, and there are several explanations offered for this behavior. One explanation is that wild canids (wolves and foxes) eat the entire animal when they catch it. Since they eat many herbivores (plant-eating animals), they end up eating a lot of , grasses and plants in the intestines of these animals. In addition, they have been known to eat certain berries and other plant material. So, dogs may eat grass because, in reality, it is a normal part of their diet.
Many times, dogs will vomit after eating grass. Did they eat grass to make them vomit? Or did they vomit because they ate grass? It is a mystery, but it seems some dogs may eat vegetation when they have an upset stomach.
The third reason-they just like it. Some dogs have certain species of grass or plant material they will search out and eat.
In any case, grass eating is basically a normal behavior, and is not of concern unless the dog does it excessively.
Article from NAPPS