To rejoice always doesn’t mean that we rejoice in the evil. It doesn’t mean that we like suffering, although we rejoice even in suffering. The basic explanation is that we rejoice in all circumstances because we have a reason to rejoice—and that reason is our Lord Jesus Christ.
Going back to the upper room, in John 15:11, Jesus says, “I have said these things to you so that My joy might be in you and your joy might be full.” So, where does our joy come from? Our joy comes from Jesus Himself and how He teaches us to view life through His Word.
I’ll give another example. Paul speaks in Romans 5:1–3 about rejoicing: “Because we are justified by faith, we have peace with God. And we rejoice in the hope of the glory of God.” We can understand that, but the next thing he says is, “And we also rejoice in our suffering.” Why? Paul goes on to explain that we can rejoice because of what God does in His people through suffering. His friction on our lives polishes our graces, and when we see that there is purpose, there can also be joy.
What destroys joy in people’s lives is when they cannot see any purpose in suffering. The human heart longs to see that there is some purpose in the suffering we go through. The Scriptures give us a whole variety of reasons why that is the case, which enables us to rejoice. We rejoice not because we are suffering, certainly not because we like pain, but because we know that God is working out His purposes in our lives.