There are many theories on this subject, but no conclusive results have been determined. Research has shown that some factors may include: dietary mineral deficiency, inadequate fiber, nausea, inflammatory bowel disease. Pet owners have reported that when their dog seems sick, he will eat grass, and then vomit. But it is inconclusive which comes first, the upset stomach or eating grass. The most likely reason a dog eats grass is that there is something about the taste that is appealing; perhaps the grass is bitter and tangy.