Some excellent insights on what Paul meant when he wrote about joy:
The more one studies these words the clearer it becomes Paul is not talking about mere happiness which certainly can be taken away by suffering, in particular extreme suffering. One clue to unlocking this paradox is that Paul says ‘joy’ is one of the fruit of the Spirit (Gal. 5.22), and indeed Paul places it only after love as one of the fruit of the Spirit. But if it is the fruit of the internal working of the Spirit in the believer, then it is not something ‘the world’ can give a person, nor can the ‘world’ take it away. It is not dependent on positive external circumstances and it may exist in spite of negative ones, but at the same time, some positive external circumstances can enhance it, and some negative ones put a damper on rejoicing–such as the loss of a loved one. One is called by Paul to rejoice in the Lord, not in one’s circumstances in any case (cf. Phil. 3.1; 4.4,10), and here he simply follows the psalmist.