Personal quantification is one fundamental mechanism of gamification. Activity trackers constitute a prototypic case for studying the psychological dynamics of this mechanism. Despite their high potential to increase physical activity, health and well-being, the effects of trackers on users’ motivation to be physically active have yet rarely been explored. The present research examines the notion that quantified feedback of gamified systems can create a dependency that can harm motivation, which becomes apparent through activity reduction when the tracker is not available. To generalize findings from experimental studies to a naturalistic setting, we examined motivational effects of activity trackers in 210 actual users through a scenario- and questionnaire-based survey. Moreover, facets of user diversity (i.e., need for cognitive closure, affinity for technology interaction, achievement motivation, and the Big Five personality traits) were taken into account. Results indicated that a decrease of motivation for physical activity in situations when the tracker is not available plays a role in everyday usage. This dependency effect was stronger for participants with high extrinsic motivation for physical activity and tracker usage, high need for cognitive closure, and low hope of success. In contrast, high intrinsic motivation for physical activity was related to a less strong dependency effect.