Why do we hold grudges

Why do we hold grudges and become resentful and unforgiving?
The people most likely to hurt us are those closest to us — our spouses,children, friends, siblings and parents. When we're hurt by someone we love and trust — whether it's a lie, betrayal, rejection, abuse or insult — it can be extremely difficult to overcome. And even minor offenses can turn into huge conflicts.
When you experience hurt or harm from someone's actions or words, whether this is intended or not, you may begin experiencing negative feelings such as anger, confusion or sadness, especially when it's someone close to you. These feelings may start out small. But if you don't deal with them quickly, they can grow bigger and more powerful. They may even begin to crowd out positive feelings. Grudges filled with resentment, vengeance and hostility take root when you dwell on hurtful events or situations, replaying them in your mind many times.
Soon, you may find yourself swallowed up by your own bitterness or sense of injustice. You may feel trapped and may not see a way out. It's very hard to let go of grudges at this point and instead you may remain resentful and unforgiving.