How can I explain the never-ending irrationality of human behavior?

We say we want one thing, then we do another. We say we want to be successful but we sabotage the job interview. We say we want a product to come to market, but we sandbag the shipping schedule. We say we want to be thin but we eat too much. We say we want to be smart but we skip class or don't read that book the boss lent us.

The contradictions never end. When someone shows up and acts without contradiction, we're amazed. When an athlete just does the sport, or when a writer just writes the words, we can't help but watch, astonished at the purity of their actions. Why is it so difficult to do what we say we're going to do?

The lizard brain.