It was proven in The Biggest Loser studies O'Malley brought up when we saw contestants having extreme changes in weight, and also saw significant changes to their metabolisms — they slowed, meaning their basal metabolic rate (BMR) was much lower than it had been before.
Extreme calorie restriction through diet and exercise slows down your metabolism, so not only will you eventually stop losing weight, but as soon as you eat a little more or exercise a little less, you'll gain weight.
Your body is smart. It's wants you to survive. It doesn't care that you want a six-pack. If you don't feed it adequate calories to sustain your body's essential functions, your metabolism will slow down so you won't lose weight.