There are a number of reasons for this.
First and foremost, the reason why we need to forgive others is because we have been told to do so. Christians are supposed to be followers of the teachings of Jesus Christ. Christ taught that people should forgive others. For example, Jesus taught the Lord’s Prayer, which talks about how we should “forgive those who trespass against us.” In other words, forgiveness is important in Christianity because it was mandated by God.
Second, we should forgive others because it is in our own best interests to do so. The Lord’s Prayer asks God to “forgive us our trespasses as we forgive those who trespass against us.” In other words, there is something of a quid pro quo here. If we do not forgive others, we are in danger of not being forgiven for our own sins. This puts our souls in danger.
Finally, it is important to forgive because hatred and anger eat away at our souls. If we do not forgive, we will instead hold on to the anger that we feel when we are wronged. If we do so, we will have a very hard time opening up to God’s love.
For these reasons, it is important for Christians to forgive others.
We forgive because we have been forgiven much. Christians believe that we have commit sins every day of our lives. And if we have come to faith in Christ, God has forgiven us of every single one of them - past, present, and future. Christ took the full punishment for each and every one of those sins. With overwhelming gratitude for the way in which we have been forgiven, we forgive others!