Jamieson, Fausset and Brown on Job 5:17

17. happy--not that the actual suffering is joyous; but the consideration of the righteousness of Him who sends it, and the end for which it is sent, make it a cause for thankfulness, not for complaints, such as Job had uttered (He 12:11). Eliphaz implies that the end in this case is to call back Job from the particular sin of which he takes for granted that Job is guilty. Paul seems to allude to this passage in He 12:5; so Jas 1:12; Pr 3:12. Eliphaz does not give due prominence to this truth, but rather to Job's sin. It is Elihu alone (Job 32:1-37:24) who fully dwells upon the truth, that affliction is mercy and justice in disguise, for the good of the sufferer.