The First Reason:

Real Forgiveness Is Costly Suffering 

 
 

"Someone may have robbed you of some happiness, reputation, opportunity, or certain aspects of your freedom. No price tag can be put on such things, yet we still have a sense of violated justice that does not go away when the other person says, 'I'm really sorry.' When we are seriously wronged we have an indelible sense that the perpetrators have incurred a debt that must be dealt with. Once you have been wronged and you realize there is just a debt that can't simply be dismissed- there are only two things to do. Pg 188 

The first option is to seek ways to make the perpetrators suffer for what they have done. … If the perpetrators suffer, you may begin to feel a certain satisfaction, feeling that they are now paying off their debt. …. Cycles of retaliation can go on for years. Evil has been done to you - yes. But when you try and get payment through revenge the evil does not disappear. Instead it spreads, and it spreads most tragically of all into you and your own character. Pg 188 

"There is another option, however. You can forgive. Forgiveness means refusing to make them pay for what they did. However, to refrain from lashing out at someone when you want to do so with all your being is agony, it is a form of suffering. You not only suffer the original loss of happiness, reputation, and opportunity, but now you forgo the consolation of inflicting the same on them. You are absorbing the debt, taking the cost of it completely on yourself instead of taking it out of the person. It hurts terribly. Many people would say it feels like a kind of death." pg 188-189 

He continues in his example stating that the person which inflicted the injustice on you will continue to have defeated you until forgive them. 

"Forgiveness must be granted before it can be felt, but it does come eventually. It leads to a new peace, a resurrection. It is the only way to stop the spread of the evil." pg 189 

'Shouldn't they be held accountable?' The wrong doers… " 'Yes, but only if you forgive them.' " pg 189 

 

"Only if you first seek inner forgiveness will your confrontation be temperate, wise and gracious. Only when you have lost the need to see the other person hurt will you have any chance of actually bringing about change, reconciliation, healing. You have to submit to the costly suffering and death of forgiveness if there is going to be any resurrection" pg 190 

"Forgiveness means absorbing the debt of the sin yourself. Everyone who forgives great evil goes through a death into resurrection, and experiences nails, blood, sweat and tears." pg 192

 
 

All text within quotes "" are from the book "The Reason for God" by Timothy Keller.