Female vampire bats regurgitate portions of their blood meals to help unfed bats in need. These food donations occur reciprocally among both kin and nonkin. This observation was a classic textbook example of "reciprocal altruism" (or reciprocity)-- the idea that cooperation is stable because cooperative investments are conditional on cooperative returns. However, this explanation for nonkin sharing has become increasingly controversial over time as various authors have proposed alternative explanations. I will review what is known about how vampire bats make helping decisions and show evidence that food sharing in vampire bats has origins in maternal care and kin selection, but is now stabilized by multiple (possibly interacting) forces of enforcement and assortment. I will highlight the underappreciated factor that the degree of fitness interdependence in social relationships can change continuously over time and that studying how cooperative relationships form provides key insights into their functions.