The problem with opportunity cost is that opportunity cost is divided among many, many things.

One percent of people will always be honest and never steal," the locksmith said. "Another one percent will always be dishonest and always try to pick your lock and steal your television. And the rest will be honest as long as the conditions are right - but if they are tempted enough, they'll be dishonest too. Locks won't protect you from the thieves, who can get in your house if they really want to. They will only protect you from the mostly honest people who might be tempted to try your door if it had no lock".

Thinking is difficult and sometimes unpleasant.

Rainy day savings are incredibly important, because from time to time, bad things happen. And if you're not prepared for that, it's going to be really terrible.

The problem with opportunity cost is that opportunity cost is divided among many, many things.

Imagine you owe on five credit cards, you owe five debts. So which debt should you pay first? And the answer is very simple: You should pay the one with the highest interest rate first. But that's not what people do.

The retail industry has its own headache: it loses $16 billion a year to customers who buy clothes, wear them with the tags tucked in, and return these secondhand clothes for a full refund.