Why Dogs Eat Grass?

Even if cats that are fed hunt less than those who have to hunt to survive, the feeling of being full and well fed does not cause a cat to give up hunting altogether. It finds the activity itself so exciting and rewarding that it continues to hunt even when it has no need to. 

This is why a well-fed cat will often kill its prey and leave it. When living with a cat, we have to accept this trait as one of its primary needs and should on no account prevent it. Cats engage in three to ten hours a day of typical hunting behaviour (locating prey, lying in wait, pouncing, killing), and this should be borne in mind when keeping domestic cats.