Because thou shalt forget thy misery, and remember it as waters that pass away:

Wesley's Notes

11:16 As waters - Thou shalt remember it no more, than men remember a land - flood, which as it comes, so it goes away suddenly.

Jamieson-Fausset-Brown Bible Commentary

16. Just as when the stream runs dry (Job 6:17), the danger threatened by its wild waves is forgotten (Isa 65:16) [Umbreit].

Matthew Henry's Concise Commentary

11:13-20 Zophar exhorts Job to repentance, and gives him encouragement, yet mixed with hard thoughts of him. He thought that worldly prosperity was always the lot of the righteous, and that Job was to be deemed a hypocrite unless his prosperity was restored. Then shalt thou lift up thy face without spot; that is, thou mayst come boldly to the throne of grace, and not with the terror and amazement expressed in ch. 9:34. If we are looked upon in the face of the Anointed, our faces that were cast down may be lifted up; though polluted, being now washed with the blood of Christ, they may be lifted up without spot. We may draw near in full assurance of faith, when we are sprinkled from an evil conscience, Heb 10:22.