Summary: Encouragement is in short supply...the church is on short rations without it.
1Now concerning the times and the seasons, brothers and sisters, you do not need to have anything written to you. 2For you yourselves know very well that the day of the Lord will come like a thief in the night. 3When they say, “There is peace and security,” then sudden destruction will come upon them, as labor pains come upon a pregnant woman, and there will be no escape! 4But you, beloved, are not in darkness, for that day to surprise you like a thief; 5for you are all children of light and children of the day; we are not of the night or of darkness. 6So then let us not fall asleep as others do, but let us keep awake and be sober; 7for those who sleep sleep at night, and those who are drunk get drunk at night. 8But since we belong to the day, let us be sober, and put on the breastplate of faith and love, and for a helmet the hope of salvation. 9For God has destined us not for wrath but for obtaining salvation through our Lord Jesus Christ, 10who died for us, so that whether we are awake or asleep we may live with him. 11Therefore encourage one another and build up each other, as indeed you are doing.
Encouragement is sometimes in short supply. I once saw a Peanuts comic strip that had Lucy van Pelt reading Charlie Brown the classic riot act after the final loss in a very long season of nothing but losses. “This is the worst team that has ever existed, Charlie Brown; we never win, and you are the most horrible manager a team could have!” With that Lucy turns and storms off. In the last panel Charlie Brown calls after her, “That’s not very encouraging!”
The late Bishop Paul Martin tells of the time when he "received a new church appointment early in his ministry. The congregation planned a reception for him on his first Sunday. During the reception, everyone came around to greet him except for one man who lingered in the rear of the room. After everyone else had greeted him, this man came forward and simply stood there. The future bishop asked him, ’What do you do here?’ The layman said, ’I look for the preacher’s weaknesses.’ And he added, ’I’m good at it! But when I find them, that’s where I get beneath him, and then I lift him up."
This defines for me the command of Apostle Paul to encourage; it is the sense of lifting one another. 
ENCOURAGING STRENGTHENS PEOPLE FOR TOUGH TIMES
People need lifting in tough times – times like we face today. Our text contains one of the “one anothers” of the Bible. Paul had said to love one another, help one another, be kind to one another, not to judge one another; here he says to strengthen or comfort one another with encouragement.
Paul’s friends at Thessalonica had tough times. They had questions about their departed loved ones, and how God was going to bring them back for the gathering of the saints at His second coming. Paul told them not to worry, they were believers, children of light, and God would remember them. He also cautioned them to put on their combat gear – faith, love and hope – and stay awake like responsible soldiers. He then told them that their faith will be strengthened as they remind each other of the fact that God hadn’t saved them to abandon them; they were his!