THIS IS SCIENCE, BABY.
Leptin is a hormone tied to your body's use of fat.
When you lose 10% of your weight, leptin falls fast. So, your stupid body thinks you're facing starvation and it tries to protect you by burning 20% less calories -- and even if you're on a diet that can make you fat.
Also, once you start losing weight the parts of your brain running your love of food have more blood in them while the part that limits eating has less.
Luckily, injections of leptin can stop the you from reacting in this way.