I think it's that we humans are designed for activity (lest we be mistaken for vegetables!), and our relatively recent habits of sitting at a computer screen or in an easy chair reading or watching the TV tell our bodies that we are conserving our energy for some reason. Thus confused, our bodies start shutting non-essential functions down, our metabolism slows, and soon, we're gaining weight and feeling sluggish and tired. A good dose of daily exercise is a great way to remind us of what it really means to be a human being -- active!