When you see the light, joy will always find expression, so if you are sad that means probably you are yet to see the light and if you want to ever remain joyful just keep seeing the light.
How do you see the light?
John 1: 1-4 1 in the beginning was the Word, and the Word was with God, and the Word was God. 2 The same was in the beginning with God. 3 All things were made by him; and without him was not anything made that was made. 4 In him was life; and the life was the light of men.
So, the light equals the word of God, just stick to the word and when understanding comes then the light will break forth – that the eyes of your understanding may be Enlighten – Ephesians 1:18.
No wonder the scripture says looking unto to Jesus the author and finisher of our faith… because Jesus is the word that gives light. John 1: 14 …And the word was made flesh.
May be the root of your sadness is because you are yet to see the LIGHT!