• Promoting health benefits is necessary but insufficient to foster sustained engagement in physical activity (PA). • Our formal decision-making model explains why health benefits hold a weak subjective value. • In this model, expected health benefits are jointly discounted by effort-discounting, delay-discounting, and beliefs distortion. • In contrast, positive affective experiences toward PA can reduce the perception of effort, provide more immediate consequences, and strengthen beliefs about health benefits. • Because affective experiences have the potential to tip the balance in favor of PA over sedentary alternatives, they should be at the core of PA promotion.