Through research and some non-fiction reading, I came across the interesting reason for dog's eating grass. There are three major reasons for this urge to consume.

Upset stomach:

Domestic dogs' ancestors used grass to induce vomiting to get rid of substance that made them uncomfortable. This is some cases would save their lives if they had, by chance, eaten something that was toxic or dangerous to the animal. It is believed that dogs have very sensitive nerve endings in their stomachs and the grass will induce the vomiting when these nerves are stimulated. Domestic dogs have kept this trait.

Need for Fiber:

Just as people chose foods that will give them the natural nutrients they need, dogs will do the same. Grass as well as many other greens are a great source of fiber. If the animal is not being given enough fiber in their daily diet, it will seek out grass to supplement for depletion.

Ancestral Urge:

In the wild, many canines eat plants as a natural and common part of their diet, though mostly indirectly. Through the consumption of prey, especially herbivores, the dogs will eat the contents of the animals stomach which includes pre-digested plants. Because grass is the most accessible green, many dogs will search this out because of a natural craving for this kind of sustenance. However, because it has not been pre-digested by another animal, some dogs will have trouble "stomaching" it. If a dog is craving grass and is eating large amounts, you can instead boil some vegetables, therefore "pre-digesting" them, and feed them to the dog instead.