Why do dogs eat grass?
Most veterinarians would agree that dogs eat grass because of some gastrointestinal symptom. It could be that eating grass will improve their digestion because the increased fiber changes the motility of the food and improves the absorption. It could be that their stomach hurts because they have intestinal parasites, a bacterial infection or are just a little stopped up. It could be to fulfill some unmet micronutritional need, even though they are being fed a balanced diet. It is not always just because they want to throw up -- less than 25% of dogs that eat grass vomit regularly after grazing.
The truth is there isn't one clear reason and eating small amounts of grass can be harmless. When grazing turn to gulping, then it's time to visit the vet. Sometimes it's behavioral. I was bored so I ate that bag of potato chips. Your dog couldn't get to the potato chips so he ate a patch of succulent grass. Thinking it will make them feel better at the time, but not necessarily for long. Pica means eating things that do not have a biologic or nutritional purpose — like paper or plastic. This could be a neurologic disease when their brain is telling them it tastes good or they have a loss of motility in an area of their esophagus. The grass then helps food get on past that area and into the stomach.
The questions to consider are does my dog gulp grass voraciously? Does he vomit? And is there pattern? (does it happen when he doesn't eat well?) If it is yes then we need to see him or her.