According to SET-POINT THEORY, we all have a well-regulated internal control mechanism that tries to maintain our present level of body fat/body weight. This all goes down deep in the hypothalamus (our brain's control center). So each time our body fat or weight drops below where our body thinks we should be, internal adjustments are made in an attempt to conserve or replenish what we lost. Our body is always trying to be in a state of homeostasis, or an unchanged "safe" state.
This is a huge reason why it ain't easy to transform your body by either gaining muscle or losing body fat, especially at an ongoing and consistent rate. YOU HAVE TO SWING HARD ONE WAY OR THE OTHER to change your body. Your body will win the physical battle... the question becomes can you win the mental battle and can you do it day after day, week after week, etc. Break the balance and you'll be rewarded for it.