"Therefore encourage one another and build each other up, just as in fact you are doing." (1 Thessalonians 5:11)
What's interesting about 1 Thessalonians 5:11 is that Paul is telling the people to do something that they are, in fact, already doing!
That's like your baseball coach saying, "Choke up on the bat and keep your eye on the ball, like you've been doing for the last ten games."
Or like your piano teacher saying, "Curve your fingers and sit up straight on the bench, like you've been doing for the last four years."
And maybe your mom and dad have said something similar, too, like, "Speak kindly to your sister, especially in the mornings as you know mornings aren’t her most agreeable times. You've already been doing this, I know, but please continue!"
Why in the world would Paul tell the Thessalonians to keep encouraging one another and building each other up if they're already doing it? For the same reason coaches and teachers and parents remind us to keep doing the good things that we're already doing: those good behaviors are producing good results, so we need to keep doing them.
We need to remember not to forget!