Why does psychosis cause emotional flattening?

Negative symptoms: One accepted hypothesis is detailed in the following: https://www.health.harvard.edu/mental-health/the-negative-symptoms-of-schizophrenia. There is too much info to fit into a 400 character response.

FLAT AFFECT: "Scientists are not entirely sure of the exact cause. It is hypothesized that it is due to differences in brain functioning—and some of the neurocognitive deficiencies that accompany schizophrenia. More often than not, flat affect is an underlying symptom of several conditions—not just schizophrenia. It may even be a side-effect of certain medications."