How to have constant Joy

From here:

God knows that complaining is an expression of our pride, and not only pride in general, but more specifically, arrogance against God Himself. Think with me. We Christians believe that God is sovereign: i.e., that in some way He controls our lives. Therefore, since we believe He either causes or allows everything that happens in our lives, complaining about what happens is an expression of our doubting His wisdom.

Puritan Thomas Watson, in a sermon on contentment preached in 1653, said, “Murmuring is no better than mutiny in the heart; it is a rising up against God. . . . The murmurer chargeth God with folly. This is the language, or rather blasphemy of a murmuring spirit: ‘God might have been a wiser and better God’ . . . Our murmuring is the devil's music.”

Do you see the subtle rebellion against God that Watson describes? Complaining is nothing less than demanding from God an explanation that suits our convenience and comfort. It is calling God to account for my expectations of how I think He ought to be and what I think He ought to do. St. Paul hinted at the danger of such an attitude when he wrote, “But who are you, O man, to answer back to God?” (Romans 9:20).

A century before Watson, John Calvin wrote, “As soon as God does not send what we have desired, we dispute against Him . . . but from what spirit is this pronounced? From a poisoned heart, as if we said, ‘The thing should have been otherwise.’ . . . It is as if we accused God of being a tyrant or a harebrain.”

Complaining is sin because it disputes the goodness of God and thus maligns His character.

St. Paul wrote, “In everything give thanks, for this is the will of God in Christ Jesus concerning you” (1 Thessalonians 5:18).You will be a more joyful Christian if you learn the secret of being free from complaining. “Rejoice in the Lord always! Again, I will say it: rejoice!” (Philippians 4:4).