This is what I call insanity - doing the exact same thing over and over and expecting a different result each time. I observe this phenomenon all the time as I watch people diet. Simply put, goals aren't reached and the diet miserably fails. With the disappointment comes depression and the person will often medicate with food and gain all of the weight back in several days. A few months later, the person tries a new diet and the cycle repeats itself.
Diets are easy to fail at because there is a law at work.
This law states that all diets work and all diets stop working. To make things simple, let me explain a little about thyroid hormones. The thyroid gland mainly produces T4 which is later converted by a deiodinase enzyme to T3. The interesting thing is that T3 is about five times more potent at raising the Basal Metabolic Rate than T4. Without the effects of the more potent T3 the Basal Metabolic Rate decreases.