He decided that the cereal didn't taste that good anymore because he actually didn't like coconut and the cereal was mostly made by coconut. Even though before Jonathan had tried tasting it and liking it a bit, he decided he didn't want to like it anymore.
His mom had bought only the coconut cereal, so to make it at least a little bit better Johathan started pouring milk on his cereal to make it taste a little bit less like coconut.
When he had his first try, Jonathan didn't like it and with his brothers, words in his head, “If you put milk on the cereal it will taste bad” Johanathan then noticed that his decision had been a bad one.
Then he finally made the decision to eat cereal without milk, because even though the cereal was bad it was better without the milk.
After Jonathan flooded the bowl with milk and the taste that he ended up was worse. Here the Cereal is the People, the Bowl is the World, the Milk is the Water, Jonathan's Brother is the other Gods and Athena, Jonathan is Zeus, the Milk Box is Poseidon.