Like Newton's Apple, gravity just is. What goes up will come down. Everything falls. Your face will fall, the leaves will fall, buildings will crumble and no matter what you do in your life, for every high, there will be a low.
That's why it's important to keep working. That's why it's important to keep putting forth your best effort. That's why it's imperative to stay hungry. You've got to keep aiming for those high points; otherwise you're just falling.