"Thou wilt show me the path of life: In thy presence is fullness of joy; In thy right hand there are pleasures for evermore." (ASV)
There is a difference between joy and happiness. Many Christians miss this concept and wonder why their lives aren't filled with happiness.
Dictionary.com gives as the primary definition for happy: "delighted, pleased, or glad, as over a particular thing." Happiness then, is dependant upon circumstances. It is an emotional state, influenced directly by the world around us.
God never promised us continual happiness, though many like to think that he did. In fact, Christ said "Verily, verily, I say unto you, that ye shall weep and lament, but the world shall rejoice: ye shall be sorrowful, but your sorrow shall be turned into joy." (John 16:20 ASV)
Clearly then, there must be some difference between happiness and joy -- else how could sorrow be turned to it? The difference is simple. Happiness is an outward thing, influenced by the world around us and therefore fleeting. Joy comes from God.
It is possible to have joy, even in the midst of terrible circumstances. Even when we mourn, we can have joy: "Blessed are they that mourn: for they shall be comforted." (Matthew 5:4 ASV) In Galatians 5:22 joy is listed as one of the fruits of the spirit. It is something that God grants to us as we draw near to Him.