Why does your dog eat grass? He might be trying to address a deficiency in his diet. Or he might have an upset stomach, and he's using grass to help him vomit. Or he might just be bored. Veterinarians don't actually know.
Another theory is based on the fact that dogs are domesticated carnivores. Dogs used to eat their prey whole, like wolves and coyotes. That means they also ingested what those animals ate – which is grass. So maybe your dog is trying to get in touch with his inner carnivore.
But don't worry. Several studies have proved that grass eating is completely normal. If your dog grazes excessively, check with your vet. And if you want to curb the behavior, just keep him busy playing while you're outdoors.