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1) When losing weight too fast you lose more muscle than fat.

2) The quantity of fat you loose can even decrease with larger calorie deficit, as your body will want to be ready in case of a long famine and will be very reluctant to get rid of its stock of fat.

3) Adaptive thermogenesis can slow down your fat loss and then make regain much more likely

4) Protein malnutrition can impact not only our skeletal muscles but even our bones (osteoporosis). In older subjects this can increase the risk of domestic accidents.

5) Not eating enough fat can have consequences on hormonal balance

6) When eating small quantities you're much more likely to have deficiencies in your micronutrients (vitamins and so on)

7) In teenagers it will impact growth and puberty

And I could certainly find many other arguments.

Generally speaking in order to lose weight fast you have to create a large calorie deficit. And unless you are exercising a lot, you could suffer from malnutrition. Its hard to get all your minerals and vitamins when you very little.

Because reducing your caloric intake too severely can lead to vitamin and mineral deficiencies, which are bad for your health. Some diets cause you to lose weight, but what you'll lose is muscle not fat. And losing weight too quickly may cause your metabolism to slow down, which makes it harder to go on losing weight.

fast weight loss can mess up the production of some hormones such as leptin and the sudden decline of those hormones puts your body in starvation mode, where it will store everything that it can and make it harder for you to lose more weight. depending on how you lost weight so quickly, it's possible that you won't be getting enough vitamins and minerals and become malnourished. it's also just really hard on you mentally and makes it harder to keep a healthy weight because it was so difficult to lose the weight that you may give up and go back to your old ways once you're down to your goal weight, whereas a gradual loss is easier to adjust your routine and lifestyle to..

Don't believe everything you read.