As for you, brothers, do not grow weary in doing good. (2 Thess 3:13)No explanation. No clarification. No new insight into how or why. Just, "Do not grow weary in doing good."
Perhaps there is additional insight from the author of Hebrews.
Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us, looking to Jesus, the founder and perfecter of our faith, who for the joy that was set before Him endured the cross, despising the shame, and is seated at the right hand of the throne of God. Consider Him who endured from sinners such hostility against Himself, so that you may not grow weary or fainthearted. (Heb 12:1-3)There it is again. "That you may not grow weary or fainthearted." But this one has some helpful insights behind it. This one starts with "we are surrounded by so great a cloud of witnesses" and moves on to "looking to Jesus." We avoid growing weary by considering Him "who endured from sinners such hostility against Himself." And the way we "run with endurance the race that is set before us" is to "lay aside every weight", specifically "sin which clings so closely". Now there's a lot of meat in that. It includes motivation and direction and the proper methods.
The truth is that many genuine believers these days may be encountering weariness. Once protected by the First Amendment, we are finding the right to the free exercise of our religion being pulled away. Once a "Christian nation", at least in name and theory, we are finding that our political system, our laws, our educational system, and even public opinion are all turning against us. All the things we trusted to have our backs are stabbing us there rather than protecting us. Who to vote for as our next representative or senator or president? Not much of a choice. Little hope that it will be someone who will agree with us. How to fight off the immorality and spiritual blight that is infecting our society? Not much available. Little hope that we can change it. What it looks like is that being a Bible-believing, Christ-following, light-shining-to-the-glory-of-God Christian will be getting us into more and more trouble as time goes on, not less and less.
Paul's words, then, are timely. "As for you, brothers, do not grow weary in doing good." And the Hebrews passage is immediately appropriate. Lay aside the weight of sins that trip us up, look to Jesus as we run with endurance, and follow His example of endurance so that we won't grow weary. These words could have been written to 21st century American Christians. And some people seem to think that the Bible is outdated.