It's likely that few would dispute that humans often make decisions when under emotional extremes that they wouldn't make otherwise. But Ariely suggests that because of our tendency to self-herd, many of our rational decisions are made as a result of extreme emotion!
If you accept the fact that humans herd (follow strangers, under the assumption that the strangers know what they are doing), it's not a great leap to understand self-herding. Who better to follow than someone whom we hold in the highest esteem (our own selves)?
As a result, when we make a decision in anger, for example, we are likely to repeat that decision under similar circumstances in the future, even under normal emotional conditions. This is because we don't recall the emotional state under which we made the original decision, but we do recall the decision! Ariely demonstrates this through a number of experiments.