Motivations and challenges jointly shape how individuals enter, persist, and evolve within software engineering (SE), yet their interplay remains underexplored across the transition from education to professional practice. We conducted 15 semi-structured interviews and employed the Gioia Methodology, an adapted grounded theory methodology from organizational behaviour, to inductively derive taxonomies of motivations and challenges, and build the Exposure–Pursuit–Evaluation (EPE) Process Model. Our findings reveal that impactful early exposure triggers intrinsic motivations, while non-impactful exposure requires an extrinsic push (e.g., career/ personal goals, external validation). We identify curiosity and avoiding alternatives as a distinct educational drivers, and barriers to belonging as the only challenge persisting across education and career. Our findings show that career progression challenges (e.g., navigating the corporate world) constrain extrinsic fulfillment while technical training challenges, barriers to belonging and threats to motivation constrain intrinsic fulfillment. The theory shows how unmet motivations and recurring challenges influence persistence, career shifts, or departure from the field. Our results provide a grounded model for designing interventions that strengthen intrinsic fulfillment and reduce systemic barriers in SE education and practice.