Why does grass grow?

What is the reason for grass growing? I could see if your yard was full of cattle, but it's not. In my opinion grass should only grow to a length which prevents the need to mow the lawn. It's as if the yard fairy keeps rubbing Rogain through out it. I'm being driven to thoughts of owning my own goats. Although I really don't want to be the guy on the street with goats...I don't want to be the goat guy. The goat guy is more pathetic then the cat lady. What annoys me on top of this whole excessive grass growing would be the fact that every time I mow the lawn a rock always seems to pop out from under the mower and find it's way to hitting me right in the forehead. To conclude...I loath grass.