Regret on social networking sites (SNS) is an under-explored phenomenon affecting users’ well-being due to negative emotions from past and present online actions. To address this gap, we investigate the causes, consequences, and situational factors of SNS-related regret. Using a grounded theory approach, we identify personal engagement (e.g., negative interactions, oversharing, social comparisons) and content consumption (e.g., excessive time spent and exposure to distressing content) as common situational factors of SNS-related regret. We further identify user-driven (immaturity, psychological habits) and sociotechnical (platform design, social influence) as causes of SNS-related regret. Our study demonstrates that these regretful experiences incite adjustments such as temporary account deactivation, stricter privacy settings, and an increased focus on offline activities. By highlighting the significance of regret in digital self-regulation, we inform platform providers and policymakers of the need for user-centered interventions. Ultimately, this paper extends our understanding of SNS-related regret and underscores its implications for healthier, more mindful digital engagement.