Thessalonians: 

(1)  Paul had preach in that city that there is another King, Jesus (Acts 17:7) and many had understood him to teach that Christ must immediately retrun.  Therefore they had given up working (1 Thess 2:9; 4:11; 2 Thess 3:8-12). 

(2) Others were concerned over the death of their loved ones, fearing that they migh have missed something since they did not live until the coming of Christ (1 Thess 4:13-18).

(3)  Timothy seems to have reported back to Paul that some were misuing their spiritual gifts (1 Thess 4:1-8)

You can move on through the other letters to see why the letters were being written. 

The Word says we show Christ our love by our obedience. He wants our love and our obedience is an overflow of that love.
"If you love Me, you'll keep My commandments"John 14:15