I used to think negativity was an amoral calibration, like pessimism. Optimism and pessimism are miscalibraions from objective probability. But there is no objective standard for positivity and negativity.
Negativity is harmful because it is defined in terms of what things aren't. You should endeavor to think in terms of what things are.
Thinking about what things are cultivates happiness and gratitude. Thinking about what things arn't cultivates resentment and anger. Negativity cultivates ontologies that guide you towards unhappiness.
Negativity also makes you dumb. Defining things in terms of what they aren't (negatively) tends is more complicated than defining things in terms of what they are (positively). Negativity obstructs you from compressing information. Compressing information equals spotting patterns in it. Negativity obstructs you from spotting patterns in information.
Negativity especially obstructs you from spotting relationships between your brain's model of you and your brain's model of external reality. Anger and hatred are premised on a clear separation between yourself and the other. It makes sense that anger and hatred would strengthen self-other separation. The negative emotions anger and hatred have obvious evolutionary advantages for your genes even as they make you stupid and unhappy.
Negativity is the opposite of mystical experience. Mystical experience is about clarity of thought. If negativity is the opposite of mysticism then negativity is incoherent thought[1].
A rational person does not choose to be unhappy. Angry people are unhappy. Rationality depends on clarity of thought. Evolution uses negativity to fog up clear thinking when it needs us to get angry. Transparency is the best disinfectant.
[1] This might reflect a literal incoherence if coherence between connectome-specific harmonic waves are behind our global workspace. ↩