Gratitude interventions can be divided into those that explicitly cultivate appreciative feelings (gratitude journaling) and those that strengthen relationships (gratitude letter). There is an absence of research on the motivation to participate in different gratitude interventions. Using an experimental approach, we compared two gratitude interventions on underlying motivations for starting and completion. We provided students (N = 904) with an opportunity to start a web-based intervention (gratitude journaling or letter). Subsequently, we measured the perceived usefulness of the intervention, social norms related to using this intervention, their self-control, and intention to start the intervention. Results showed that keeping a gratitude journal and writing a gratitude letter to someone were perceived as equally useful and socially acceptable. Yet participants felt less efficacious in writing a gratitude letter, which in turn decreased self-initiation and the actual completion of the intervention. As for individual differences, people with greater dispositional gratitude expected the intervention to be easier, more beneficial, and socially acceptable; meaningful sex differences also emerged. Our findings provide new insights into underlying motivations and individual differences that influence the initiation and efficacy of gratitude interventions.