The broaden-and-build theory is a prominent framework in positive psychology that considers positive emotions as essential for people to flourish. The theory does not specify the origins of positive emotions during development, although experiences in the first years of life are considered influential for long-term adjustment and well-being. In this article, we propose a developmental extension of the broaden-and-build theory. This extension identifies belonging as a potential third function of positive emotions in addition to broadening and building. We suggest that positive emotions, inextricably linked to social interactions, play a crucial role in satisfying infants' and young children's fundamental need to belong. The belonging function of positive emotions might pave the way for broaden-and-build processes. Our proposed extended framework provides an opportunity to consider the origins and functions of early positive emotions in the context of social interactions.