We examined relations among select components of the 4-H Thriving Model, focusing on youth sparks and four presumed determinants: program quality, developmental relationships, situational engagement, and dosage. Based on our results and results of previous studies, we propose a formal theory of youth program sparks. Three hundred fifty-six Texas 4-H youth from a variety of 4-H programs state-wide completed a questionnaire measuring sparks, program quality, developmental relationships, situational engagement, and 4-H program dosage. We measured two indicators of thriving: growth mindset and hopeful purpose. Sixty-eight percent of study participants reported being female, 28% reported being male, 1% reported being nonbinary/third gender, and 0.28% indicated being transgender. School grade levels ranged from sixth grade (n=4) to "graduated from high school" (n=21). We found significant linear relations between sparks and three determinants: program quality, developmental relationships, and situational engagement. The relation between sparks and dosage was curvilinear. We found significant linear relations between sparks and thriving. Combined with results of previous studies, we propose a theory of youth program sparks. Using Zetterberg's (1965) framework for theory construction, we propose scientific (Aristotelian) definitions of key concepts and propositions about relations among those concepts. The theory provides a basis for future research that can inform youth development policy.