Adaptability. It's a wonderful mechanism we have to eventually cope with life's changes.
However, when dieting or exercising, adaptability is not our friend. It's that awful plateau we hit when we are trying to loose weight and after a great week, that darn scale shows no change. It's running the same distance on the same road, and the clock shows no change. Our bodies have an incredible ability to adapt to the changes that we make and "go with the flow." Your body is always trying to maintain homeostasis or balance.
When dieting or exercising, our bodies react to the changes we make and make adjustments (loosing pounds, building muscle). But eventually, or bodies adapt to these changes and reach that balance point.
When dieting, you need to change your program and yo-yo your caloric intake. When exercising you need to constantly vary the type of workouts and increase intensity to achieve your desired results