I Timothy 2:1
   I exhort therefore, that, first of all, supplications, prayers, intercessions, and giving of thanks, be made for all men;

   We are to pray for ALL MEN, yet the church epistles demonstrate the Apostle Paul's practice and example of thanking God especially for the believers.  Thanksgiving should always be directed toward that which has already been given to us by God.  The believers have been given much in Christ and give much as they walk with God.  GOD deserves our thanks, for what believers have been given and for what believers do as they walk with Him.

I Corinthians 1:3-4
   Grace be unto you, and peace, from God our Father, and from the Lord Jesus Christ.
   I thank my God always on your behalf, for the grace of God which is given you by Jesus Christ;

   Believers have received much in Christ by the grace of God.  We are so thankful to God for all He has given us in Christ Jesus by His lovingkindness and by His grace.

2 Thessalonians 2:13
   But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth.

   Our brothers and sisters in Christ are beloved of the Lord.  We have an obligation, we are bound, to thank God for them - God loves them!  He loves them so much that He chose them from the beginning.  We are thankful to God for His love for the believers and for the fact that He chose them.  God has saved the believers.  God has set them apart from the world (sanctified them) by giving them holy spirit and making them His sons and daughters.  We are thankful that God has given salvation and the gift of holy spirit to men and women who have believed the truth.  The simplicity of Romans 10:9-10 is something for which I am very thankful.  That a man or woman can confess with their mouth the Lord Jesus and believe in their heart that God has raised him from the dead and at that moment go from being just body and soul to being sons and daughters of God with holy spirit within and eternal life - WOW!  We should be thankful for the believers whom God has chosen.

Romans 1:8
   First, I thank my God through Jesus Christ for you all, that your faith [believing] is spoken of throughout the whole world.

   Believing is acting on God's Word.  We should be thankful to God for the walk of believers.  It is God in Christ in them.  God has given them His Word and they have believed it!  We are thankful for the believers who walk forth acting upon God's Word.  The Internet brings new meaning to "your believing is spoken of throughout the whole world."  We are thankful to God that there are men and women out there believing His Word.

I Thessalonians 2-4
   We give thanks to God always for you all, making mention of you in our prayers;
   Remembering without ceasing your work of faith [believing], and labour of love, and patience of hope in our Lord Jesus Christ, and the sight of God and our Father;
   Knowing, brethren beloved, your election of God.

   Remember God chose the believers!  In our prayers we are so thankful as we remember the work believers produce by believing God, the labor [fatiguing work] believers produce by loving God, and the patience [faithful continuance in the Word] believers produce by the hope given by God.

Colossians 1:3-6
   We give thanks to God and the Father of our Lord Jesus Christ, praying always for you.
   Since we heard of your faith [believing] in Christ Jesus, and of the love which ye have to all the saints.
   For the hope which is laid up for you in heaven, whereof ye heard before in the word of the truth of the gospel;
   Which is come unto you, as it is in all the world: and bringeth forth fruit, as it doth also in you, since the day ye heard of it, and knew the grace of God in truth:

   Believing, love, and hope bring forth fruit in the lives of the believers as they walk in the grace of God.  We should give thanks ALWAYS to God for the believers and remember in our prayers their walk of believing, love toward one another, and hope for the return of Christ.

Philippians 1:3-7
   I thank my God upon every remembrance of you,
   Always in every prayer of mine for you all making request with joy,
   For your fellowship in the gospel from the first day until now;
   Being confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ:
   Even as it is meet for me to think this of you all, because I have you in my heart; inasmuch as both in my bonds, and in the defense and confirmation of the gospel, ye are all partakers of my grace.

   We should offer thanksgiving to God every time we remember the believers.  We are thankful for the believers' continuance in the Word of God and that God will continue to work in them as they walk until Christ returns.  It is a necessity that we think this way about them, hold them in our hearts, and are thankful to God for the believers as we are all partakers of the grace of God.