ilove Diets put your body into a feast and famine mode.
So you are a bit fat, you think 'I am going to lose this weight'. You cut your calories, your body responds by slowing your metabolism because it thinks it is in a famine stage, you use less calories in order to save what you can, if you hold out for long enough you will burn fat stores.
However, this whole time your body is screaming at you to eat, so you will eventually break and eat something fatty/salty/sugary (millions of years of evolution cannot be overcome with willpower alone). You body thinks Great! Feast time, right better put all that fat back and add some more because the famine will be back soon.
You are fat again (maybe an extra kilo or two on where you started in the first place), so you think 'I am going to lose this weight' and the cycle starts again, each time adding a little extra fat to your body for the next famine.
I know all of this because I was very fat for a very long time and ate very little for extended periods, I eventually managed to lose 75kgs but it took over 5 years and went very very slowly and I have to eat rather a lot. I don't eat many "modern" foods. Predominantly meat and veg and fruit - very very little in the way of grains etc. Even thinking about going on a diet now is enough for my body to react to.