Listen to part of a lecture on this topic in a psychology class.

Think about when you were a kid. Imagine you like this one playground. You play there a lot, have lots of fun, you know. Okay. Now imagine that one day, for no apparent reason, your parents decide that they don't want you playing there anymore. You are not allowed to go there anymore. Of course you're not gonna like that one bit. It's not fair. And now that you are not allowed, you want to play there even more than before. So you sneak over there anyway. You go to this playground despite your parents' rules.

Here's another example.

There was a town that passed a law that banned the sale of a certain kind of soap. There was an ingredient in this soap that was harmful for the environment. So stores weren't allowed the sell the soap anymore. Keep in mind that this ingredient had no effect whatsoever on this soap's ability to clean things. None. But people found out about the upcoming restriction and got upset. They thought they should be able to buy whatever soap they wanted. It wasn't right to take this soap away. And a week before the law went into effect, what happened? People went and bought a whole lot of this particular soap, way more than they would have in another circumstance.