The core work of the soul is to develop her ratzon (desire to attach to her spiritual source). The fiery desire of the soul is described as a burning flame lifting herself up and seeking to bond with her eternal root. While doing so she touches other souls and ignites them without detracting from herself.
Indeed the main work of the soul is to shine light onto shadows and penetrate darkness. For that she needs tremendous power; the driving force being her ratzon (her inner desire & longing) to illuminate. Without the existence of darkness light would go unnoticed and unappreciated.
Since she originates from the higher realms she is awarded with the ability to fight her way through all sorts of blackness. Through holding tight to Hashem, the giver of light, no matter what challenges come her way, she is fed boundless light. What keeps her grip tight? Her ratzon to light up and spread goodness onto others.
The soul is named a candle since this is her essential nature; to fortify her desire each day anew, no matter how difficult it is. The stronger her will the greater the light she is given and stronger her impact on dispelling darkness.