This is a secondary analysis study using raw data from the 8th Korea National Health and Nutrition Examination Survey (2019), which identified the factors affecting the happiness of middle-aged adults. The data of the 1,862 subjects from the said survey were analyzed. Complex sample cross-analysis and complex sample logistic regression were performed on middle-aged adults. Studies have shown statistically significant differences in education level, marital status, household income quintile, economic activity, smoking, sleep time, subjective health awareness, and stress awareness. Education level, smoking, subjective health awareness, and stress awareness were found to be factors affecting the happiness of middle-aged adults. Compared to the poor health groups, happiness was 3.41 and 3.20 times higher in the good and usual health groups, respectively. The group that felt less stress was 6.15 times happier than the group that felt more stress. The happiness of elementary, middle school, and high school graduates was 0.16, 0.72, and 0.47 times lower, in this order, than that of college graduates and above. Smokers’ level of happiness was 0.29 times lower than non-smokers.