Converging evidence shows that the cultural importance of romantic love has increased dramatically with societal development; it now serves as a primary criterion for choosing partners, a foundation for family formation, and a pervasive theme in cultural production. Yet here we show that this rise does not continue indefinitely: romantic love peaks and then declines at the highest levels of development. This rise-and-fall pattern emerges both cross-nationally (9,474 participants across 45 countries) and diachronically (automatic annotations of 13,409 U.S. movies, 1971–2020). This poses a puzzle: if romantic love sustains long-term pair-bonds, and if safer, more affluent environments make such bonds more viable, why would love decline where it should thrive? We resolve this paradox by proposing that what declines is not love itself, but the need to signal it. Using a formal model and cross-national analyses (N=7,333 and 84,382), we show that when long-term commitment becomes common, overt romantic signaling becomes unnecessary and even counterproductive: the most committed individuals benefit from not signaling, because doing so would reveal that they expected the relationship to need reassurance. In affluent societies, love becomes quieter not because bonds are weaker, but because they are more secure.