Digital platforms such as social networks, e-commerce portals, streaming applications, online learning systems, and mobile apps depend heavily on user engagement for sustained growth and long-term success. Behavioral theories help explain why users engage, how engagement is sustained, and what psychological mechanisms influence digital interactions. This paper provides a comprehensive conceptual review of Behavioral Theories of User Engagement in Digital Platforms, integrating cognitive, emotional, motivational, and social dimensions. By synthesizing classical theories—such as Self-Determination Theory, Flow Theory, Uses and Gratifications Theory, Social Influence Theory, Cognitive Load Theory, and Habit Formation Theory—with modern digital behavior models, this study proposes a holistic Behavioral Engagement Framework. The findings highlight key drivers of engagement such as intrinsic motivation, perceived usefulness, social affirmation, habit loops, reward systems, and personalized experiences. The article concludes with implications for design, ethics, and future research.