Jamieson, Fausset and Brown on Job 6:5

5. Neither wild animals, as the wild ass, nor tame, as the ox, are dissatisfied when well-supplied with food. The braying of the one and the lowing of the other prove distress and want of palatable food. So, Job argues, if he complains, it is not without cause; namely, his pains, which are, as it were, disgusting food, which God feeds him with (end of Job 6:7). But he should have remembered a rational being should evince a better spirit than the brute.