According to the United Nations’ World Happiness Report, Japan’s happiness level is not very high, and the freedom to make life choices tends to be limited. Since the 1970s, “why happiness does not necessarily correlate with income levels” has been an important topic in the study of happiness. In this study, we conducted a survey of 20,000 Japanese nationals and analyzed the respondents’ data by using income, education, health, human relations, and self-determination as explanatory variables. The results show that the sense of wellbeing in relation to age drops at around midlife, generating a U-shaped curve, while the sense of wellbeing does not increase in proportion to an increase in income. The findings also indicate that, following health and human relations, self-determination is a stronger determinant of a sense of wellbeing than either income or educational background. It is believed that self-determination in life enhances motivation for and satisfaction with the action chosen and ultimately leads to an increased sense of wellbeing. It is noteworthy that those high in self-determination have a high degree of happiness in Japanese society where the freedom to make life choices is deemed to be narrow.