The difference between shallow happiness and a deep sustaining joy is sorrow. Happiness lives where sorrow is not. When sorrow arrives, happiness dies. It can't stand pain. Joy, on the other hand, rises from sorrow and therefore can withstand all grief. Joy, by the grace of God, is the transfiguration of suffering into endurance, and of endurance into character, and of character into hope--and the hope that has become our joy does not (as happiness must for those who depend upon it) disappoint us." Walter Wangrin, Reliving the Passion
"Christians rejoice even while they truly sorrow - - because their rejoicing is in the hope of heaven....While joy overcomes sorrow, it does not put an end to it."  John Calvin 

Joy in the bible is the means by which we are sustained in suffering - - - - - - - and joy is that for which suffering is preparing us..... John Piper

"Your sorrow itself shall be turned into joy. Not the sorrow to be taken away, and joy to be put in it's place, but the very sorrow which now grieves you shall be turned into joy. God not only takes away the bitterness and gives sweetness in it's place, but turns the bitterness into sweetness itself." Charles Spurgeon (on John 16:33)