On a late Saturday evening, 5 of my friends and I went to a Yakiniku restaurant. Being the growing boys that we are, we all ordered all you can eat meat. However, this would prove to be deadly.
In the first hour, we plowed through plate after plate and had consumed a very big amount of meat. Miraculously, we were still hungry so right before the last order, we asked for a final round of meat. We ordered multiple plates thinking we could easily finish the food.
However, by the time the meat came out, our brains had gotten the signal that we were full. Panic struck in as we looked at the plates of meat in front of us. We began to cook the meat and one by one, reluctantly eating each piece. After what had seemed like an eternity, we were down to our final pieces. We had done it, but we were on the verge of spewing our insides. As we uncomfortably swallowed our final pieces of meat we realized we had experienced diminishing marginal utility. We had gone from enjoying every single piece of meat, to feeling nauseous and barely being able to think of even eating the meat.
You see, as delightful as Yakiniku can get, when you push yourself to consume more than your body wants, your body stops wanting it. You begin to hate it and this is diminishing marginal utility. Just like with all products a producer sells or provides, too much consumed by one consumer, and they are bound to experience diminishing marginal utility.

A picture from the evening
Diminishing marginal utility is the only reason why stores allow their costumers to eat all they want, because while the consumers think they are getting a great deal, the producers know that they will only be able to eat so much before their bodies starts to reject the food. Its just natural for our body, even with non-food products, to get bored with something after playing too long with it.