This study aimed to explain the process underlying the “criteria for sustainable travel choice among tourists of Generations Y and Z” and to uncover the role of individual–cultural differences in these decisions. The attitude–behavior gap in sustainable travel was the core problem. Using a grounded theory approach (Strauss–Corbin model), data were collected through semi-structured interviews and field notes. The sample comprised 18 participants from Generations Y and Z. Data were analyzed through open, axial, and selective coding, and validity was ensured via member checking with narrative summaries and a four-option agreement scale. “Choice criteria” emerged as the central phenomenon, shaped by environmental and social motivations (causal conditions) within a context of individual and cultural differences (contextual conditions). Financial, infrastructural, and informational challenges (intervening conditions) moderated the intensity and direction of the process. Actors’ strategies—grounded in social and personal responsibility—translated into tangible selection criteria: environmental, social–local, economic–well-being, value congruence, and informed decision-making. The overarching outcome is social acceptance and the normalization of sustainable travel. The study’s paradigm model explains the shift from “intention” to “behavior” among Generations Y and Z and accounts for the attitude–behavior gap. To facilitate sustainable behaviors, the study recommends reducing perceived costs and risks, strengthening infrastructure and access to sustainable services, and leveraging social media purposefully to normalize destinations and foster value congruence.