James gives the second reason in verse four: "... that you may be perfect, complete, lacking nothing." (ἵνα ἦτε τέλειοι καὶ ὁλόκληροι ἐν μηδενὶ λειπόμενοι.)
But before he gives the second reason, he gives a second command: "Let endurance have its perfect work." (ἡ δὲ ὑπομονὴ ἔργον τέλειον ἐχέτω)
What does it means to "let endurance have its perfect work?" Think of the 10k marathon. If a runner gives out after 9k, his endurance did not complete or finish the job. Endurance "has" its perfect work, when it makes it all the way to the finish line. That's what endurance is supposed to do: take you the distance.
Here's James' point. When you're still in pain, or you're out of a job, or you're still not sleeping well, or your situation is getting worse not better, or all of the above are true simultaneously ... don't quit trusting God! Don't jump off the Potter's wheel! Continue affirming and trusting in God's goodness, wisdom, faithfulness, and sovereignty.
Easy to say!! Sure it's easy to say, and Yes, it's teeth-clenchingly difficult. But that is what James is saying.
But HOW do you "let endurance have its perfect work?" Just mindlessly mantra Romans 8:28?!! No ... but to answer the how question will require a separate post ...
So ... A key reason not to give up and the second reason we should could it all joy when we fall into various trials is God is using them to make us perfect and complete, lacking nothing.
I think it is a mistake to try to distinguish between "perfect," "complete," and "lacking nothing" here. James is piling on the synonyms for effect -- like we do when we say it is a wonderful, fabulous, glorious day.
What does perfect mean? It doesn't mean God is fixing our minds, so they think without logical error. It doesn't mean God is fixing our bodies, so that they are always hale and hearty.
"Perfect" in James describes the kind of gifts that come down from the Father of Lights (James 1:17), the law of liberty (James 1:25), and the man who is able to bridle his whole body (James 3:2). The variety of items James describes as "perfect" makes it a bit difficult to determine precisely what he has in mind.
Perhaps it is best to allow the other two synonyms he uses to focus his idea for us: complete and lacking nothing. The perfection God is working in our lives is a completeness where nothing that should be present is lacking. That sounds like what Paul describes as "the measure of the stature of the fullness of Christ" (Eph. 4:13). In other words, full Christlikeness of character.
Now, unless you are really enthused about gaining spiritual maturity or full Christlikeness, learning that your trials are helping you become fully like Jesus won't incline you to "all joy." And, frankly, that is a major part of our problem. We have forgotten that being a disciple of Jesus means making being like him the ultimate and focal object of our life (Mat. 10:24-25).
When we do long to be like Jesus more than we long to be like anyone or anything else, then knowing that God is perfecting us into the image of His Son will be a grounds for great joy.