It is common to think that if your dog or cat is eating grass it is a sign they are sick or there is a nutrient lacking in their diet that they are naturally craving. The latter is partially true. In Jack Sommars article "Why Does My Dog Eat Grass" on PetsMatter.com, he explains that wolves, the dogs ancester, are drawn to grass because it has intestinal parasites that will help control worms. However, because of modern medicine your pet does not need to eat grass to control worms. Therefore we theorize that the habit of eating grass is instinctual, a characteristic brought down from the wild.