Sexual desire is associated with romantic relationship satisfaction and maintenance, yet desire for a partner often declines over time. Self-expansion (new experiences that facilitate growth) with a partner boosts desire, but how this occurs is not well-understood. Across three studies—a 21-day daily experience study, a one-month weekly experience study, and an experimental study—we tested whether closeness, and a new construct otherness (seeing a partner in a new light), accounted for the association between self-expansion and desire. Across studies, self-expansion was associated with higher closeness and otherness, and, in turn, higher sexual desire (indirect effect through otherness significant in Studies 1 and 3). The findings provide evidence for the importance of fostering closeness, as well as otherness, in the maintenance of desire.