Why do dogs try to eat grass

There are a number of theories but no hard scientific evidence. One theory is that it acts as a natural laxative/purgative. If they eat enough they will vomit. This may be true in some instances but does not explain the cases where a dog eats a small amount all of the time. A theory for these is they gain some nutritional benefit from eating the grass. In any case there does not appear to be any harm to a dog eating grass.