Why do people help others on mobile shopping sites, even when members have never met? To answer the research question, this study applies attachment theory to investigate the relationships between formation variables (trust, enjoyment, and timesaving), transfer variables (site, group, and interpersonal attachments), and an outcome variable (reciprocal altruism). An online survey was conducted of shoppers who had purchased tourism products with smartphones. The results show that trust, enjoyment, and timesaving have significant effects on site attachment, which, in turn, have a significant effect on both group and interpersonal attachments. Interpersonal attachment also has a highly significant effect on group attachment. Reciprocal altruism is influenced by both site and interpersonal attachments. The findings not only provide theoretical contributions to the tourism literature but also practical contributions to the tourism industry.