The story of The Fall is convoluted. Biblically, an Archangel fell from hierarchy on purporting to take the place of God. Subsequently turning into a serpent, tempting Eve to fall from her position of purity and take Adam down along with her. The Archangel concerned was, among other things, known as the Bringer of Enlightenment, and while the Tree of Knowledge was the one sporting the fatal apple, the Tree of Life was central to the force of existence. Once privy to the Knowledge bestowed by the fruit of said Tree, we (humans) came to know the difference between good and evil, which we didn't know before. One train of thought associates the serpent with salvation. That's ecclesiastical argument for you.
A positive outlook contributes to our ability to do things, to sense clearly, to view with optimism. A heavy heart, on the other hand, leads to inertia, confusion and a tendency towards cynicism. In line with the wave equation familiar in quantum physics, we move through these states temporarily, with corresponding peaks and troughs. Nothing is forever, after all. When we fall, we inevitably get up again.
In certain situations, we can maintain a sunny disposition for a long time, but inevitably we must eventually drop back to a less vibrant version of being in order to avoid 'burning out'.