No one can say for sure why........mine go through spells of eating loads of the stuff and then they don't bother with it for a few weeks. I notice they especially go for it after it has been raining. I walk dogs and they all eat it on and off, some more than others. One dog I used to walk would eat it and then throw up. Maybe there is something in the grass that they need in their diet? Does grass help them to digest their meals? or settle their stomachs if they feel a little queasy? Does it just taste nice? Who knows it is one of those mysterious things that only a dog would be able to tell you.......
There is some more info about this topic on this thread here: http://www.dog-obedience-training-re...ats-grass.html