Answer:
This saying properly goes: The grass always looks greener on the other side of the fence. This is Existentialism for cattle. In human terms, it means that what is out of reach or unavailable or taboo may appear of greater worth than what is present and real and allowed, or in short that people often desire what they cannot have while despising what they do have.
The grass is greener means its going to be juicier and nicer tasting :)