Matthew 12:35 A good person produces good things from the treasury of a good heart, and an evil person produces evil things from the treasury of an evil heart.
When I read this I thought, "What we keep in our heads and what comes from our mouths eventually drops down into our hearts." To be good, we have to think on things which are good. Philippians 4:8 tells us this, "Fix your thoughts on what is true, and honorable, and right, and pure, and lovely, and admirable. Think about things that are excellent and worthy of praise." When we do this our hearts become good and our actions follow.
When we dwell on upset and talk about it and talk about it and talk about it... our thoughts become angry, then our hearts... then our words and actions.
To be produce good things from a treasury of a good heart we must put good things in our mind so that good falls down into our heart!! This takes effort on our parts.. it is a decision we make. It is a choice we have. To be good we must take our thoughts captive (2 Corinthians 10:5). Once we take our thoughts captive...our hearts and our actions will follow.