So, in a naïve attempt to preserve fuel (fat) the body starts to slow down its metabolism and then starts to burn (and consequently reduce) your muscle mass instead of your fat. And your metabolism and your muscle mass are the very two things that you are going to need for long term stable weight loss. Meanwhile, you're tempted by every sugary carb that comes within a hundred yards.
So can you see the paradox that you can get into by using diets for quick weight loss? Physiologically, it's like being trapped between the "devil and the deep blue sea". You're trying to lose the fat, but your body won't let it go. In such a struggle, sooner or later something is going to give - either your waist line will expand, or your overall health is going to deteriorate.
And one last word of warning: if you do decide to do a diet for quick weight loss it is important that you try and maintain you bodies requirements for other nutrents such as vital vitamins and minrels