In the Salvation Process, we encounter good and bad, blessing and trial and prosperity and testing.  All are designed by God as part of the molding process that He is invoking to build the character of His Son in us.  Notice this meaning from the commentaries:
 

1 Thess 5:18

[In everything 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. ~Clarke

1 Thess 5:16-18

 Everything. All circumstances, even hardships and affliction. This, though singular, seems to embrace the three commands of 5:16-18. God's will includes constant joy, ceaseless prayer, and boundless thanks, attitudes made both necessary and possible in Christ Jesus.  ~ Wycliffe

Here are the three commands of verses 16-18:

Rejoice evermore. Pray without ceasing. In every thing give thanks: for this is the will of God in Christ Jesus concerning you. ~1 Thess 5:16-18

In everything-even what seems adverse; for nothing is really so (cf. Rom 8:28; Eph 5:20.) So Christ (Matt 15:36; 26:27; Luke 10:21; John 11:41).

This-that ye should "in every thing give thanks," "is the will of God in Christ Jesus (the Mediator in whom that will is revealed: cf. Phil 3:14) concerning [toward: eis (NT:1519)] you." God's will is the believer's law. Put commas after each of the three precepts (1 Thess 5:16-18), making "this" refer to all three. ~  Jamieson, Fausset, and Brown

If we pray without ceasing, we shall not want matter for thanksgiving in every thing. As we must in every thing make our requests known to God by supplications, so we must not omit thanksgiving, Phil 4:6. We should be thankful in every condition, even in adversity as well as prosperity. It is never so bad with us but it might be worse. If we have ever so much occasion to make our humble complaints to God, we never can have any reason to complain of God, and have always much reason to praise and give thanks: the apostle says, This is the will of God in Christ Jesus concerning us, that we give thanks, seeing God is reconciled to us in Christ Jesus; in him, through him, and for his sake, he allows us to rejoice evermore, and appoints us in every thing to give thanks. It is pleasing to God.  ~ Matthew Henry

Lesson 29: Instructions – Give thanks
 
 1] In all circumstances, even hardships and affliction, give thanks to God.
 2] Understand that God allows blessings as well as trials in our life.
 3] Rejoice always.
 4] Pray without ceasing.
 5] Know that God's will is being carried out in your life.

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