Job 21:6 (ESV)

6 When I remember, I am dismayed, and shuddering seizes my flesh.

In fact, Job himself is terrified by his thoughts and by what he is about to say. He is about to argue that people do not always get what they deserve. The wicked is not always punished; therefore, it is not so obvious that suffering is punishment for wickedness. And yet, by arguing in this way, Job knows that he will come close to denying the justice of God’s ways. This makes him tremble with fear.