One can have different reasons to lose fats. Some want to lose weight as it would improve their appearance and other want to lose weight to become fit and healthy. Losing weight takes time and is considered a long term goal. There are many fast weight loss programs available. But they are unhealthy and temporary. You tend to regain your weight after stopping that program. Also many people want to lose fats instantly as fast is not good enough for them.
You can lose fats if your calorie burning capacity of your body is greater than your consumption of calorie. But problem arises as this type of diet cause muscle loss along with the fat loss. It is been shown by research that a typical human body can lose a maximum of 2 pounds of fat in a week. So if your diet is causing more weight loss than that means it is causing you to lose muscles and water.
Also this type of dieting can reduce your body's metabolic rate. If you eat less food your body's primary instinct of survival kicks in. This causes you to slow your metabolic processes and conserve energy. Hence the rate of loss of weight becomes very slow. So fewer fats can be burned by this process
Muscle also helps in losing weight. So as more and more muscles are lost by this instant dieting the body's metabolism takes further hits. So as a result the metabolic rate of the body becomes all time low. This is the reason that those who opt for fast diets regain their original weight and more. So the only way to lose fats with no potential risks is to it slowly over a long period of time.