Rephrasing the third and fourth verse:
"We cannot help it, we must praise the Lord when we hear how you Thessalonians, who are enduring so much persecution and trouble, are growing exceedingly in your faith and your love for one another."
Persecution did not diminish their faith, instead their faith "grows exceedingly" they cling more intensely to Christ.
Also their love is growing. Persecution drew the members closer together.
But now compare the introduction of this second letter with the first letter, for something is missing.
In the first letter Paul is thankful for their faith, love and steadfastness of hope.
In the second Paul is thankful for their increasing faith and love, but there is no mention of their hope.
They are steadfast or patient under the continuing persecution, which implies that there is hope, but apparently the Thessalonians are starting to wonder "where is justice",.
Paul affirms them telling then that their unshaken faith in all their persecutions indicates that God judged them to be worthy to partake in God's eternal kingdom of righteousness.
Their hope and faith is directed to the second coming when everything will be made right.
The letter doesn't give much hope that things will get better before then, indeed it will get worse as the "man of sin" will arise and in pretended role of being the "voice of God" will cause many to be lost. But hope and faith will continue in steadfast patience in those anchored in the love of truth and faith of Jesus.
There will be justice
everlasting life in the kingdom of God for those who in steadfast patience continue in faith and love,
everlasting destruction for those who obey not the gospel of our Lord Jesus Christ.