Why should I forgive? (Short question:short answer)

Q: Why should I forgive someone when they don't apologise?

 A: Grace and the cross teaches us something about why it is important for us to forgive.  Besides God's glory what, does the Cross of Christ show us.  It shows us God giving us what we don't deserve, mercy.  Because of our sin against God, God's justice requires a penalty, that penalty is eternal death, but because God loves us He wanted to give us a way out,ba pardon.  Through God's mercy (the Cross) we can now partake of this pardon, even though we do not deserve it.  You see someone had to pay the penalty; what Gods justice requires, but no man was good enough to pay, except for one, Jesus.  Now through Jesus getting what He doesn't deserve we can have what we don't deserve, Salvation from the penalty of our sin.  This is what we are doing when we forgive someone of a trespass they commit against us.  Because they did something to us, that action requires a just penalty (in human terms we call this revenge) just like our sin against God.  Forgiveness is our way to give that person what they don't deserve, the same thing Jesus gave us, a pardon from the penalty of that action.  When we forgo vengeance through forgiveness we are echoeing the very thing Christ gave us....Mercy.  Just as God put our just deserts on Christ; we are to put the other person's just deserts upon God for He says "....
, “Vengeance is Mine, I will repay,”....."(Romans 12:17-19)
 I believe throgh Christ death, God showed us Mercy trumps  justice.  When reasonable we are to always show and search out mercy before justice.  This is one of the many lessons of the Cross.

Psalm 103:10-12
He has not dealt with us according to our sins, nor punished us according to our iniquities. For as the heavens are high above the earth, so great is His mercy toward those who fear Him; as far as the east is from the west, so far has He removed our transgressions from us.
Isaiah 43:25
I, even I, am He who blots out your transgressions for My own sake; and I will not remember your sins.
Matthew 6:14-15
For if you forgive men their trespasses, your heavenly Father will also forgive you. But if you do not forgive men their trespasses, neither will your Father forgive your trespasses.
Luke 24:46-47
It was necessary for the Christ to suffer and to rise from the dead the third day, and that repentance and remission of sins should be preached in His name to all nations.