Social Functionalist Theory (SFT) emerged 20 years ago to orient emotion science to the social nature of emotion. Here we expand upon SFT and make the case for how emotions, relationships, and culture constitute one another. First, we posit that emotions enable the individual to meet six "relational needs" within social interactions: security, commitment, status, trust, fairness, and belongingness. Building upon this new theorising, we detail four principles concerning emotional experience, cognition, expression, and the cultural archiving of emotion. We conclude by considering the bidirectional influences between culture, relationships, and emotion, outlining areas of future inquiry.