In 2017, South Korea officially became an aging society when the proportion of people aged ≥65 years reached 14.2% of the total population. Furthermore, the proportion of older adults living alone showed 24.4% of the total one-person households in 2017 .
Some studies have suggested that older adults living alone not only experience depression, poorer health, less social capital, loneliness, increased prevalence of social isolation, reduced social support, lower quality of life, and less subjective happiness than those living with others, but they are also more likely to feel sad, hopeless, and worthless [2, 3, 4]. Conversely, many older adults value the independence, familiarity, personal comfort, and privacy of living alone and age just as successfully or even better than those who co-reside [5, 6].
The happiness of older adults living alone warrants attention because they are more vulnerable to the adverse effects of social isolation than older adults living with their families due to greater risk of losing their spouses and friends, which at the same time makes them more dependent on available social capital. Happiness is a broad concept that includes various related concepts, such as life satisfaction, a good life, a better life, well-being, and quality of life [7−13]. A happy life is a basic human right; quality of life is an objective condition, and happy emotions are subjective evaluations. Various factors, including depressive symptoms, health status, social support, and social network, have been understood to affect the happiness of older adults living alone [5, 7]. Most studies on the happiness of older adults living alone limited their focus to the influence of health and social support, although some authors adopted a broader approach. For instance, Kim  suggested that household income, depression, subjective stress levels, subjective health levels, quality of life, and lack of required medical services influence the happiness of adults aged 65 years and over and living alone. Van Leeuwen and colleagues  identified nine quality-of-life domains for older adults (mean age 71−91 years) living alone; these include autonomy, role and activity, health perception, relationships, attitude and adaptation, emotional comfort, spirituality, home and neighborhood, and financial security. The World Health Organization  identified health and social services, behavioral determinants, personal determinants, physical environment, social determinants, and economic determinants as factors affecting happy and active aging . Such findings suggest that social participation, housing environment, and health status are the major factors influencing happiness in older adults living alone. In listing the factors influencing happiness in older adults, it is also important to understand the structural relevance of these factors.
Older adults' participation in social activities is an essential element of successful aging . Through their participation in social activities, older adults can fulfill their appropriate social roles, form friendly relationships, engage in productive activities, prevent loneliness, and increase happiness . Kim and Ha  suggested that sports, volunteer activities, and travel are productive leisure activities that enhance physical health and reduce suicidal thoughts in older adults living alone. The idea that the health and well-being of individuals who reside in specific environments for prolonged periods are influenced by their neighborhood environment, regardless of their personal characteristics, is one of the most widely tested hypotheses [18, 19]. Older adults spend most of their time without any specific purpose in the community; therefore, they might be affected by their local environment and surrounding social networks . Stahl and colleagues  reported that a social community network based on traditional relationships between neighbors used as a poor alternative support system for older adults living alone contributed to solving their various difficulties. Furthermore, Putrik and colleagues  found that the neighborhood characteristics influenced the subjective health and depression levels of residents. They divided neighborhood characteristics into physical (e.g., buildings and public transport) and social (e.g., social ties, stability) characteristics.
Above all, older individuals wish to maintain good health during aging. Quality-of-life and happiness depend largely on health status; a disease or disability can affect independence by limiting individual behaviors . Therefore, subjective health awareness is considered a major factor influencing happiness .
In general, it is desirable for older adults to achieve happiness by maintaining connections with the social community while receiving physical care, emotional support, and financial support from their families [22, 23]. However, older adults living alone are very vulnerable to experiencing a lack of support from their kinship community; therefore, obtaining support through social resources and non-kinship support systems may help them enjoy a healthy and happy life. To enhance successful aging, it is important to identify the multidimensional factors affecting happiness in older adults living alone and empirically identify the relationship between these factors and the health of older adults. Based on the relevant literature, the factors influencing the happiness of older adults living alone can be largely divided into personal and environmental aspects.
This study developed and tested a model to explain and predict happiness in older adults living alone in Korea and developed a hypothetical model for happiness in these individuals. In this regard, we propose a structural model that explains and predicts happiness in older adults living alone by verifying the fit of the hypothesized model. Finally, we investigated the direct, indirect, and total effects of factors affecting happiness in older adults living alone by evaluating participants' subjective feelings regarding their health and their objective interactions with the environment.