Diminishing Marginal Utility?

I had noticed that my dog tends to let his dogfood sit in the bowl for a long time. At first I thought he did not like his dog food. I bought another brand of dog food and noticed that he still lets the food sit there.
I though maybe I am giving him too much food. I started giving him half a cup of dog food in the mornings instead of his usual one cup of dog food. After a few days I noticed that he is eating everything in his bowl.
Next I noticed he is willing to roll over and do various tricks for his plain dog food. This would not have happened when he was getting one cup in the mornings.
He is getting one and one half a cup now, more than before, and he loves it more!
Is this Diminishing Marginal Utility of dog food or something else?
Is Diminishing Marginal Utility biological?