Joy is life, expressing. The free flow of life energy is what you call joy. The essence of life is Oneness -- unity with All That Is.

This is what life is: unity, expressing.

The feeling of unity is the feeling that you call love. Therefore, in your language, it is said that the essence of life is love.

Joy, then, is love, expressing freely.

 Whenever the free and unlimited expression of life and love -- that is, the experience of unity and oneness with all things and with every sentient being -- is prohibited or limited by any circumstance or condition, the soul, which is joy itself, is not fully expressed.

Joy not fully expressed is the feeling that you call sadness.
Friendship With God
Neale Donald Walsch
Page 264