During life, mental and physical negativities arise as afflictions to distract us from it, due to the ego's grasping onto the illusion of permanence.
These negativities leads to suffering due to the Five Poisons of desirous attachment, aversive anger, and delusive ignorance, and their daughters, pride born out of ignorance and desire, and jealousy arising attachment and aggression.
The Five Poisons are the destructive emotions.
Through meditation and mindful practice of wisdom, the charitable acts of kindness to help, and the loving sacrifice in service to other sentient beings aid in satisfying the ego, pacifying it.
Once the ego is pacified, the mental and physical negativities are replaced with the antidotes of love, compassion and insight,which are strengthened through practice and training.
Buddha Nature represents the essential nature of the mind, which is pure and undefiled by the destructive emotions.
When joy comes easily, and the mind is easily calmed through meditation, the antidotes have succeeded in curing it of the Five Poisons.
For that joy arises due to Buddha Nature, the seed of Enlightenment itself.
Thus Enlightenment is always with us, like the pure white light of freedom.
The Gandavyuha Sutra: http://en.wikipedia.org/wiki/Avatamsaka_Sutra#The_Gandavyuha_Sutra