Dogs may eat grass just because it tastes good. They can even look for a certain type of grass that is their favorite. Sometimes it looks like my own dogs think they are cows. They like to have a grass snack when I let them out into the backyard.
Another reason is that dogs may behave a little like their wolf relatives from a very long time ago. Most wolves eat animals that feed on grass, and find the stomach contents very tasty.
Grass can contain extra vitamins or minerals. You might want to check to be sure your dog is on a good food and maybe add some green vegetables. Dogs should never eat grass that has been treated with weed-killer or fertilizer, because it could make them sick.
Lastly, some dogs eat grass because they have an upset stomach. If they ate something that bothers them, grass can help make them throw up. This may be OK if it only happens once in awhile. But if your dog suddenly starts eating grass (especially if they are doing it a LOT), it would be a good time to take them to the veterinarian for a check-up.
Don't you wish that dogs could talk and explain this themselves? I'll bet they wonder why we don't eat grass, too.