Why is Good Friday Good?
My best answer may be similar to the words in this Puritan poem:

My Father, enlarge my heart, warm my affections, open my lips, supply words that proclaim 'Love lustres at Calvary.'
There grace removes my burdens and heaps them on thy Son,
made transgressor, a curse, and sin for me;
There the sword of thy justice smote the man, thy fellow;
There thy infinite attributes were magnified,
and infinite punishment was due,
and infiinite punishment was endured.
Christ was all anguish that I might be all joy,
cast off that I might be brought in,
trodden down as an enemy
that I might be welcomed as a friend,
surrendered to hell's worst
that I might attain heaven's best,
stripped that I might be clothed,
wounded that I might be healed,
athirst that I might drink,
tormented that I might be comforted,
made a shame that I might inherit glory,
entered darkness that I might have eternal light,


My Saviour wept that all tears might be wiped from my eyes,
groaned that I might have endless song,
endured all pain that I might have unfading health,
bore a thorned crown that I might have a glory-diadem,
bowed his head that I might lift up mine,
experienced reproach that I might receive welcome,
closed his eyes in death that I might gaze on unclouded brightness,
expired that I might for ever live.


O Father, who spared not thine only Son that thou mightest spare me.
All this transfer thy love designed and accomplished; 
Help me to adore thee by lips and life.
O that my every breath might be ecstatic praise,
my every step buoyant with delight, as I see
my enemies crushed,
Satan baffled, defeated, destroyed,
sin buried in the ocean of reconciling blood,
hell's gates closed, heaven's portal open.


Go forth, O conquering God, and show me the cross,
mighty to subdue, comfort and save.

(Taken from the book The Valley of Vision)