Market theory does not fully explain the economic choices we make. Commentator Wim Hordijk says we must also look to behavioral economics and evolutionary psychology to understand the economy.
Research into behavioral economics has shown, for example, that our assessment of what something is worth to us can be directly, and predictably, influenced. This is the illusion of the free lunch, something humans are known to fall for even when economic theory would clearly suggest we select a more valuable option at a small cost.
Ariely also beautifully elucidates how we sometimes operate on social norms, while other times we fall into market norms. The difference is in whether there is a price attached to something.
If a friend invites you over for dinner, she will probably appreciate it if you bring a nice bottle of wine along (social norms). However, if instead you slap $20 (the price of a nice bottle of wine) in cash on the table and say "thanks for a lovely dinner," she would most likely be offended (market norms). Mixing social norms and market norms inappropriately often leads to irrational behavior and, possibly, even to conflict or misunderstanding.
Our irrational behavior is not just random though. The scientific experiments are repeatable. Each time we are faced with a similar situation, we tend to behave in a similarly irrational way. So, next to the bad news that we are not nearly as rational as we might have thought (or hoped), there is also good news in that we can understand and predict our irrational behavior, at least to some extent. This, in turn, can help us improve our decision making and change our behavior for the better. In other words, we can try to be more rational about our irrationality.