Recent studies on growth mindset have noted their significant benefit to students, in both supporting higher academic achievement and being correlated with greater levels of persistence in STEM disciplines. In spring 2023, the first national survey of the impact of student facilitation of informal physics outreach programs on student development was distributed to Society of Physics Students chapters. This survey, which received 704 responses, noted an important connection between self-reported measures of students' growth mindset and their engagement in informal physics outreach programs. This work explored the relationship between having a growth mindset and engaging in outreach--specifically, whether a growth mindset encourages students to participate in outreach or if outreach itself fosters a growth mindset. To investigate this, we examined student responses to three open-ended questions, looking for indicators of mindset and directionality. Across 239 student responses, we identified 26 instances of mindset, with 24 reflecting a growth mindset. Notably, 17 of these indicated that outreach had contributed to its development. Through thematic analysis, we identified five significant themes: more to learn, confidence in ability/knowledge, students' realization of understanding, development of understanding, and recognition of difficulty. Four of these themes emerged from responses demonstrating a growth mindset, while the last was observed in fixed mindset responses. These findings, drawn on self-reported data, strongly suggest that participation in informal physics outreach programs can provide students with meaningful opportunities to develop a growth mindset, which may significantly benefit their academic performance and motivation to complete their degrees.