1. They should rejoice always in appreciation of God's righteous character.
The Lord is my strength and my shield; my heart trust in Him and I am helped; therefore my heart exults, and with my song I shall thank Him (Psalm 28:7).
2. They should have constant joy out of appreciation for Christ's redemptive work, which derives from a gracious, loving, merciful, and compassionate God (1 Peter 1:8-9).
3. They should rejoice in appreciation of the Holy Spirit's ministry on their behalf (Acts 10:44, Romans 14:17, Romans 8:14-27).
4. Believers should rejoice always because of the vast array of spiritual blessings they posses (Ephesians 1:3-40).
5. Believers should have joy in God's providence as He orchestrates everything for their benefit (Romans 8:28-30).
6.They should be joyful out of gratitude for the promise of future glory (Psalm 16:8-11 and Matthew 5:12).
7. Answered prayer should always be a source of joy (John 16:23-24).
8. Believers in the Messiah should always be joyful out of appreciation for the gift of God's word (Colossians 3:16).
9. The privilege of genuine fellowship should always bring joy to the believer (1 Thessalonians 3:9).
10. True believers cannot help but express their joy at the saving proclamation of the Gospel (Acts 15:3).
The MacArthur New Testament Commentary 1 & 2 Thessalonians p.184.