"But Timothy has just now come to us from you and has brought good news about your faith and love. He has told us that you always have pleasant memories of us and that you long to see us, just as we also long to see you." NIV translation
Paul is rejoicing in the good report of Timothy concerning the Christians in Thessalonica. As a body of believers, we can rejoice at the triumphs of the other parts of the Body and hurt with and for the parts that are struggling.
"Therefore, brothers, in all our distress and persecution we were encouraged about you because of your faith." NIV translation
Nobody wants to work day after day and year after year without any sign of accomplishing something. Therefore, even when it was difficult, Paul and his team were encouraged by the fact that the efforts produced results (fruit).
In John 15, Jesus said that if we remain in Him we will bear fruit.
We do not always get to see that fruit immediately but when we do we can be encouraged just as Paul was in this verse. It can also remind us to ask ourselves if we are producing any fruit and if we are not to get back to Jesus as that is the only way that will produce a harvest.
"For now we really live, since you are standing firm in the Lord. How can we thank God enough for you in return for all the joy we have in the presence of our God because of you?" NIV translation
We truly live when we bring others to knowing and trusting in Jesus. Paul was filled with joy because of the fact that people's lives were changed and they were living by faith in God. It never gets old seeing people turn to God and being saved from hell. That, my friends, is really living!
"Night and day we pray most earnestly that we may see you again and supply what is lacking in your faith." NIV translation
At this time, they did not have a Bible like we do today and so it was harder for them to gain knowledge. Paul simply wanted to teach them more about their faith in Jesus Christ.