When a monkey loses a banana to a rival, he feels bad, but he doesn't expand the problem by thinking about it over and over. He looks for another banana. He ends up feeling rewarded rather than harmed. Humans use their extra neurons to construct theories about bananas and end up constructing pain.
Loretta Graziano Breuning, Habits of a Happy Brain: Retrain Your Brain to Boost Your Serotonin, Dopamine, Oxytocin,  Endorphin Levels
brain, neuroscience, psychology