Graduate Teaching Assistants (GTAs) and Undergraduate Learning Assistants (ULAs) play vital roles in undergraduate education, particularly in large STEM courses, by fostering professional relationships with undergraduate students and supporting their academic success. Despite this importance, GTAs and ULAs often begin teaching with limited training. Therefore, practicing self-reflection can be considered beneficial for both GTAs and ULAs. It is by engaging in self-reflection that they are more likely to consistently assess and improve their teaching methods. As such, this study investigates how GTAs and ULAs engage in self-reflection to develop professional relationships with undergraduate students, through using the Vitruvian Model of Reflective Practice (VMRP) worksheets, semi-structured interviews, and a survey. The impact of significant yet simple affective domain gestures, such as remembering undergraduate students' names, in developing professional relationships was revealed. Moreover, in enhancing such relationships, other strategies such as body positionality, addressing undergraduate students' misconceptions, and providing reassurance emerged as crucial factors for developing professional relationships with undergraduate students. In addition, a survey was designed and distributed among undergraduate students to assess their perception of relationship development approaches suggested by the GTAs and ULAs and identify additional methods provided by the undergraduate students. Methodological transparency and informal discourse were highlighted by undergraduate students as actions for strengthening their professional relationships with both their GTAs and ULAs. Overall, this research provides valuable insights into the importance of the affective domain in educational settings and offers practical recommendations for improving the professional relationships between assistants and undergraduate students.