Paul, Silas and Timothy always pray for them and thanks God for their faith and their love and patience. The love of the Thessalonians had for each other was known throughout the land. However, the writers urged them to show more love in (1 Thessalonians 4:10). They asked the Lord to cause the love of the Thessalonians to increase (1 Thessalonians 3:12). As a result, the love of these Christians there was more than it was. God answered the prayer. That is why Paul and his friends thanked God. Many Christians miss aspects of God's love for them. This is probably the reason for the prayer.