The NASA/IPAC Teacher Archive Research Program (NITARP) partners small groups of educators with a research astronomer for a year-long authentic research project. This program aligns well with the characteristics of high-quality professional development (PD) programs and has worked with a total of 103 educators since 2005. In this paper, surveys were explored that were obtained from 74 different educators, at up to four waypoints during the course of 13 months, incorporating data from the class of 2010 through the class of 2017. This paper investigates how participating teachers describe their motivations for participating in NITARP as evidenced in these feedback forms. Analysis of self-reported data allows a mapping onto a continuum ranging from more inward-focused to more outward-focused; there is a shift from more inward-focused responses to more outward-focused responses. This insight into teacher motivations has implications for how the educators might be supported during their year with the program. This work provides a new way of parameterizing why educators participate in PD programs that require a considerable investment of time. NITARP, since it has many qualities of successful PD, serves as a model for similar PD programs in other STEM subjects. Likewise, the analysis method might also be useful to similarly evaluate other PD programs.