Each person's body has a different sweet spot for weight that it tries to maintain. Francesco Carta fotografo/Moment via Getty Images For decades, people have been told that their weight problems can be solved by math: Calories in, calories out. If weight were a simple math equation, more people would likely be the weight they desire. But it is much more complicated. There are several theories as to why it is difficult to lose weight. Some focus on genetics and metabolism while others claim envi