Adam Clarke on 1 Thessalonians 5:18

In every thing give thanks - For this reason, that all things work together for good to them that love God; therefore, every occurrence may be a subject of gratitude and thankfulness. While ye live to God, prosperity and adversity will be equally helpful to you.

For this is the will of God - That ye should be always happy; that ye should ever be in the spirit of prayer; and that ye should profit by every occurrence in life, and be continually grateful and obedient; for gratitude and obedience are inseparably connected.