What is the reason fast diets don’t work?

I was wondering because they say many people who are on fast diets gain their weight back and sometimes even more.

I’m curious as to why. Is it something your body does? Like I hear if you don’t eat for a while your body goes into shock and the next time you eat it stores most of the food just in case you go for another long time before eating
or
Is it because people go back to their old habits?
or
Is it something else?

http://ping.fm/MDTgJ