The On and Off Switch for Mental Tranquility

What goes up must come down— Newton's law of gravity. But gravity is not responsible for the mental hem and haw, from contentment to discontent, back and forth, again and again. We worry about the future, feel guilty about the past, and are too often consumed with dark thoughts. Some people seem blessed with an easy-going demeanor able to accept their circumstances without resisting. But even those lucky few experience nights of tossing and turning. What is it in the human mind that disrupts the tranquility of our soul?