Wearable activity trackers hold great potential for facilitating self-regulated health behavior, thereby improving physical fitness and preventing cardiovascular diseases. Unfortunately, many users discontinue tracking after only a few months, mitigating large-scale health effects. To identify usage barriers and psychological mechanisms resulting in tracker abandonment decisions, it is essential to characterize former users regarding their abandonment reasons as well as former tracker usage patterns. Within the present research, we reviewed past literature on wearable activity tracking attrition and developed an online questionnaire for assessing abandonment reasons. Results from 159 former users revealed insights into the relative importance of abandonment reasons, former users’ usage patterns, evaluation of personal quantification, and tracker acceptance. Correlational analyses showed that intensity of tracker usage and data interaction, current physical activity, and tracker acceptance were related to specific abandonment reasons. Moreover, abandonment due to perceived data inaccuracy/uselessness and loss of tracking motivation were linked to negative attitudes towards personal quantification. Furthermore, permanent abandonment decisions were particularly related to loss of tracking motivation. Based on the results, we derived six design guidelines for supporting continued tracker usage.