My Simon loves grass but he throws it up. Still goes back and eats more while I have to clean it up.

When I go outside with him the first thing he wants to do is eat grass. Then he doesn't throw it up outdoors. He waits until he is inside to do it.

What is it about grass they like or need? My Chianti doesn't seem to need it. She won't touch it and no throwing up for her.

I give them hairball formula Science Diet so why does Simon need to throw up?