Purpose: Service user involvement, which implies involvement in decision-making processes important for organizing, developing, and evaluating health care, is sparsely investigated from the perspective of adolescents and young adults (AYAs) with cancer. Thus, the purpose of this qualitative study was to investigate AYAs' reasons for participation in service user involvement initiatives during their cancer course. Methods: Data from 12 semistructured interviews with AYA cancer patients and AYA cancer survivors aged 16-29 years were analyzed using thematic analysis. The participants had been involved in one or more service user involvement initiatives in "Kræftværket," a youth support center and social organization for AYAs with cancer in Copenhagen, Denmark. Results: Four main themes with matching subthemes were identified: Community (Being together, Developing together, and In the same boat), Normality (A new normal and Something to do), Contribution (Something at heart, Support to other AYAs, and Improving the health care system), and Codetermination (Real participation and Influence on one's own everyday life). Conclusion: Participating in service user involvement initiatives had a positive impact on the illness course of AYAs with cancer as the participation recreated a feeling of normality, codetermination of one's own everyday life, and met a wish for altruism in terms of helping others.