The causes of forgiveness come from experience of being forgiven before. Before the Bishop was a Bishop he was someone who was trying to find holiness in his life too. Bishop before was too much like Valjean, very materialistic. This was before the Bishop had seen the poor and the sick struggle near his church he had changed his state of mind into being a bit more forgiving. Jean forgives Javert, because he has been through the changes in life and has experience, and remembers people like Napoleon that had helped him. Valjean does not seem to be forgiving his mistakes as he only provides the negative side of the crimes he had committed. Forgiveness is comes from experience and having dealt with forgiving someone or being forgiven.
