Instead of being united under Jesus, the people became divided by their loyalties to another. They dwelt on their ill feelings, becoming more and more angry. The Ephesians needed to be reminded that their mission on earth was to follow Christ and to share the news of his sacrifice with others. A big part of this mission was to build up other believers and support them in their faith. Christ taught his followers to love each other as he loved them.
Forgiveness is a natural part of that love.
Evil speaking is not. Paul wanted to see the Christians in Ephesus building each other up instead of tearing each other down. He knew that this was the best way to continue the love shown by Christ. That should be the real motivator for our actions, not a list of do's and don'ts. Of course, when Jesus spoke of being bread, he was using the bread as a symbol for something else.