John 16:21

A woman when she is in travail hath sorrow, because her hour is come: but when the little son is born, she no more remembers the anguish, for joy that a man is brought into the world.

Revelation 7:14-17

And I said to him, Sir, thou knowest. And he said to me, These are persons who are come out of great tribulation, and have washed their robes, and made their robes white in the blood of the Lamb.

Treasury of Scripture Knowledge

Summary

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

Bible References

Because

John 16:21
A woman when she is in travail hath sorrow, because her hour is come: but when the little son is born, she no more remembers the anguish, for joy that a man is brought into the world.
Revelation 7:14
And I said to him, Sir, thou knowest. And he said to me, These are persons who are come out of great tribulation, and have washed their robes, and made their robes white in the blood of the Lamb.