Research from multiple domains suggests that individuals benefit from having other people in their lives who endorse an incremental mindset, believing in humans' capacity to change and improve through effort. We hypothesize, however, that people high in incrementalism may have unrealistic expectations about the ease of change, leading them to become frustrated with others who promise change (change-strivers) but achieve only partial success. In a longitudinal study, change-strivers made promises of change to their romantic partners and attempted to keep those promises over two weeks. Romantic partners who were higher in incrementalism were initially more optimistic that change-strivers would successfully change, but subsequently more distrustful toward change-strivers whose change attempts failed. Furthermore, partners high in incrementalism were more likely to attribute failure to the change-striver's lack of effort, rather than to the difficulty of the behaviors. The findings highlight circumstances when incremental mindsets may have costs in relationships.