Research suggests that when asked to reason about another person, people tend to say the person ought to form motivated, unrealistic, overly optimistic beliefs. Why do people say others ought to deviate from the accurate and realistic evidence? One possibility is that people intuitively treat different mental states, like hope, as dependent on belief. Under this view, or what we call the belief-dependent thesis, people correspondingly prescribe overly optimistic beliefs to others because they think they are necessary to experience the proper amount of hope and its benefits. We find mixed support for the belief-dependent thesis as a way of understanding why people prescribe motivated reasoning to others. Across two studies, we demonstrate that people believe it is much easier to maintain motivational mental states, like hope, when one has an unrealistically optimistic belief compared to when one has a realistic belief. However, we do not find that individual differences in perceived belief dependency predict people’s likelihood of prescribing overly optimistic beliefs to others. We discuss how these results may help explain the ubiquity of motivated reasoning, and we explore potential directions for future research.