This chapter proposes a sociological theory for loneliness, which has primarily been studied at the individual level. The authors draw on sociological theories, life course theories, and a simplified socio-ecological model to introduce five key axioms that explain how loneliness evolves over the life course. These axioms emphasize the role of human agency, early life conditions and experiences, the timing of life events, linked lives, various forms of capital, and broader community and societal contexts. Through this lens, loneliness is not just an individual feature but also a feature of social structure, shaped by a complex interplay of personal characteristics and influences at multiple levels. The chapter highlights how the immediate environment and larger societal structures can both directly and indirectly affect the loneliness trajectory. Applying a sociological perspective to loneliness research enhances our understanding of the contextual factors that influence loneliness across time and space, supports the examination of interactions between micro (individual), meso (community), and macro (societal) levels, and provides a framework for designing interventions that target societal and community factors, rather than focusing solely on individual solutions. This approach offers a comprehensive understanding of loneliness that is informative for more effective, systemic strategies for prevention and reduction.