"for we walk by faith, not by sight." 2 Corinthians 5:7
"Now faith is the assurance of things hoped for, the conviction of things not seen." Hebrews 11:1
If we walked by sight, we would claim that what was seen was all there was to the world. I do not mean this merely in the naturalistic sense, but also in the sense that we can see the wrongness of the world. If we walked by sight alone, we would think that all the evil was unalterable--that it was irredeemable.
Faith does not say "All is right with the world," but rather "All will be right with the world." And it will, when our faith becomes sight; when what we now hope for--the redemption of all things--finally arrives, and becomes present for all to see. Until that day we have the assurance that the world has not yet ended--there is more to it than this--and what is to come is the reconciling all things to God, through Christ, the making all things right, to the glory of God the Father, Son, and Holy Spirit, and therefore, for our good.
Faith implies joy, but it does not imply not mourning, rather, it implies eager longing for that day when every tear will be wiped away. In the present time, if the world were to simply end, then it would lie in contradiction, but we have a foretaste of the resolution to the contradiction in Jesus Christ our lord, who reconciled us to the Father in his body. Yet we await the fulfillment of this, when all will be made right, and until then we mourn the present state of things, pointing to the fact that something must be done to make the world right again, just as God says when he does in fact make the world right again. Thus our mourning is agreement with what God has promised to do. To mourn is to say "God ought to do something about this" just as he has promised to do, though perhaps no in our measly small way we think we would like.