Richard P. Bentall makes convincing argument that happiness is a psychiatric disorder.† Notably Caesar was said to prefer the company of fat men, with the implied causation that happy -> fat. One also sees this in the weight gain usually caused by SSRIs. At the same time obesity has well measured effects on many metabolic processes, so it is likely there is some effect on the brain.
† [PDF] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1376114/pdf/jme...