"Now God himself and our Father, and our Lord Jesus Christ,
direct our way unto you.
"And the Lord
make you to increase and abound in love one toward another, and toward all
men, even as we do toward you:
"To the end
he may stablish your hearts unblameable in holiness before God, even our Father,
at the coming of our Lord Jesus Christ with all his saints." (1 Thessalonians 3: 11-13)
and
"Furthermore
then we beseech you, brethren, and exhort you by the Lord Jesus, that as
ye have received of us how ye ought to walk and to please God, so ye
would abound more and more." (1 Thessalonians 4: 1)
and
"And the very
God of peace sanctify you wholly; and I pray God your whole spirit and
soul and body be preserved blameless unto the coming of our Lord Jesus Christ.
"Faithful
is he that calleth you, who also will do it. " (1 Thessalonians 5: 23-24)
Paul had nothing but good to share with his Thessalonian fellow-believers, and even in the fourth chapter, when Paul refutes unequivocally that the false teaching that the resurrection has already occurred.
What did Paul pray for?
He wanted his Thessalonians to abound in love, which we cannot help but expound as we keep receiving His gifts of grace and righteousness (Romans 5: 17)
It is God who establishes us in righteousness (Isaiah 54: 14) and grace (Hebrews 13: 9), and the knowledge of this we must revisit every day, as being in this world, we are tempted to forget who we are, or we are tempted to think that we must do something out of an evil spirit of unbelief or fear. Faith works through love (Galatians 5: 4), so Paul wrote then and exhorts us now to abound in this love.
Last of all, he called upon the Lord to preserve us, body, soul, and spirit. He has our spirit already, and the more that we rest in the sacred and secure certainty of His love for us, with greater joy we can work out His salvation in our lives with great wonder (Philippians 2: 12)