My name is ***** ***** I am an experienced small animal veterinary surgeon, I will be very pleased to work with you today and will try my best to answer your question to your satisfaction.
It is actually not unusual for many dogs to enjoy eating grass and it does not always do them harm
in fact one of my dogs often did this throughout her life.
Why dogs eat grass is not clear it could well be just because they just like to, or because in the past herbivores such as rabbits would be part of a dog's natural diet and they would swallow the intestinal contents of their prey as well so grass would part of their natural diet which modern day dogs try to emulate.
As long as it not affecting Cosmo's health then I would not worry about it too much ..