Lightness is like hitting a spring of water inside. A sudden break out from the dryness of your normal reactions as you begin to express from within, rather than just responding to what you see outside.
It usually happens when you come upon a quality that hasn’t been destroyed or jaded by time, something very old and inherent in you that has survived. Most qualities and talents are developed through use, but are also in some way diluted and changed. Lightness is touching what makes you unique, because it has never changed.