Dogs eat grass for several possible reasons.
There is no single explanation that fits every dog.
Are dogs trying to make themselves vomit?
That is the popular theory.
Some dogs do vomit after eating grass.
But many show no signs of illness before grazing, and most do not vomit afterward.
Grass eating by itself is therefore not strong evidence that a dog has an upset stomach.
Dogs can eat plant material normally
Dogs are not strict carnivores.
They can digest a mixed diet, and plant material can be part of normal exploratory or feeding behavior.
Grass also contains fiber.
Some dogs may simply like it
Anyone who has watched a dog carefully choose tender new grass has reason to suspect taste and texture matter.
Boredom can contribute too.
A dog outside with little else to do may graze because the grass is available.
When should grass eating concern you?
Occasional grazing is usually not a problem.
A sudden major increase deserves more attention, especially with repeated vomiting, diarrhea, weight loss, weakness, appetite changes, or abdominal swelling.
There is also a practical hazard.
Lawns can contain pesticides, fertilizers, herbicides, parasites, or toxic plants.
Does grass eating mean a dog needs fiber?
Not automatically.
Dogs on complete diets can still eat grass.
You cannot diagnose a nutritional deficiency from grazing alone.
The same rule applies to many dog behaviors: one behavior rarely has one perfect translation, just as tail wagging does not always mean happy.
For many healthy dogs, an occasional mouthful of grass is simply normal dog behavior.
And that’s the short explanation.
Related explanations: Why do dogs wag their tails?, Why do dogs tilt their heads?, and Do dogs dream?