Background: There is growing recognition in health policies and professional guidance that youth-friendly services must include the values and views of young people in their healthcare. The term “engagement” has become increasingly used in literature to recommend that healthcare professionals should involve young people in participatory methods and include them in their decision-making. Yet, the engagement of young people within health interactions remains a complicated process, often influenced by lived contexts, value systems and lifestyle choices. Successful engagement of young people is often cited to ameliorate health-related behaviours, improve health outcomes and increase awareness of their health needs; yet, a paucity of research exists for healthcare professionals seeking to engage young people effectively in healthcare interactions. Study Aim: To explore and define young people’s engagement within their healthcare interactions. Methods: A grounded theory study was conducted over a six-month period to gather young people’s perceptions of their health interactions. Results: The grounded theory study identified that young people produce engagement-related behaviour by means of an interpretive process. Three interconnected theoretical categories emerged: (i) prejudgement, the beliefs with which young people enter into health interactions; (ii) learning to be a patient, the means by which young people learn from their interactions to develop in-context engagement-related behaviour; and (iii) validation, the selective interpretation of information to justify their perception of the interaction. The core category identified that young people demonstrate a reliance on affect heuristics within healthcare interactions, which may influence the extent young people feel able to engage with healthcare professionals. This was elevated into a substantive theory of affect-mediated engagement. Conclusion: Findings suggest that a dual-process perspective of cognition may be useful to understand how young people engage in their healthcare; this could potentially be used by healthcare professionals to target issues that impact on a young person’s engagement. This study’s findings form a basis which would benefit nurses, occupational therapists and other healthcare professionals in developing person-centred interactions that empower young people in becoming stakeholders in their own health.