Despite reflection being identified as a key component of behavior change, most existing tools do not explicitly design for it, carrying an implicit assumption that providing access to self-tracking data is enough to trigger reflection. In this work we design a system for reflection around physical activity. Through a set of workshops, we generated a corpus of 275 reflective questions. We then combine these questions into a set of 25 reflective mini-dialogues. We deliver our mini-dialogues through MMS. 33 active users of fitness trackers used our system in a 2-week field deployment. Results suggest that the mini-dialogues were successful in triggering reflection and that this reflection led to increases in motivation, empowerment, and adoption of new behaviors. Encouragingly, 16 participants elected to use the system for two additional weeks without compensation. We present implications for the design of technology-supported dialog system for reflection.