This study was conducted to explore the factors that influence the development of happiness during childhood using longitudinal data obtained from the Korea Youth Panel Survey(KYPS). Specifically, the causal relationships between factors impacting the individual children, the home environment factor, and the happiness of children were examined over a 3-years-period. The subjects evaluated in this study included 2,844 children (1,524 boys and 1,320 girls) and 2,844 parents who were administered the KYPS. The data were analyzed using the SAS program. The results revealed that happiness that develops during childhood remains stable and constant, which indicates that prior happiness has a strong effect on future happiness. The individual factors affecting the children, which include schoolwork achievement and extra private education, were found to have a great influence on the development of happiness at all ages. The use of longitudinal data in this study is a new method in the field of Human Development.