Often asked, but unless we can ask the dog there's no definitive answer.The most popular explanation is that the dog does it because of worms or indigestion (the two may be related; the worms may cause discomfort in the stomach ).Sometimes the dog vomits soon after eating the grass, which seems to be evidence for this theory.
Some dogs appear to be herbalists.One of mine is eating stinging nettles at present, not something she does all the time, just recently.She also, on occasion, takes to eating hogweed (ordinary, not the giant hogweed) and sometimes goldenrod..She is quite fussy, plainly hunting for just the right age and type of hogweed leaf.All of mine pick blackberries, neatly plucking the ripest and ignoring the rest.They must know something about plants that we don't! They are even fussy about the grass, not eating the stuff in the garden, preferring the longer grass in the horse paddock, especially when it's wet.