It's one of the most common questions vets get, and the honest answer is nobody knows for sure why dogs do this. A few things seem to be going on though:

• Upset stomach: Some dogs eat grass to help themselves vomit when they're feeling sick.
• Boredom: Dogs left alone in the yard for long stretches will graze the way a bored person might snack.
• Diet gaps: Some vets think dogs missing certain nutrients or fiber go looking for it in plants.
• Just because: Plenty of healthy dogs eat grass now and then with no clear reason at all.

If your dog is eating grass occasionally and seems otherwise fine, it's usually nothing to worry about. Frequent grass-eating paired with vomiting or lethargy is worth a vet visit.