From John Piper’s A Godward Life:

Matthew 13:20-21: “As for what was sown on rocky ground, this is he who hears the word and immediately receives it with you; yet he has no root in himself, but endures for a while, and when tribulation or persecution arises on account of the word, immediately he falls away” (RSV). Joy that evaporates in the heat of affliction was not joy in God, but in comfort. Paul said that he knew God had chosen the Thessalonians because, “You received the word in much affliction, with joy inspired by the Holy Spirit” (1 Thessalonians 1:6, RSV). Joy in tribulation is the evidence of joy in God, not ease.