Dogs are not known for being fussy eaters. Put it in your dog's path and he'll gobble it up, whether it be table scraps, garbage ... or grass.
Dogs are primarily carnivores (meat-eaters). Although they like to eat meat, they can also survive on a well-balanced vegetarian diet: Cats, on the other hand, may die without animal protein. Like all living creatures, dogs need a combination of fats, carbohydrates, proteins, vitamins, minerals, and water in a balanced diet that provides enough calories to meet their daily needs.
We're not sure why your pup likes grass, although there have been many theories offered. Primarily, dogs are descended from wild canids (wolves and foxes), which ate the entire "kill" when they hunted for food. Since they consumed many herbivores (plant-eating animals), they wound up eating a lot of plants and even berries found in the stomach and intestines of their prey.